BUG: Series.plot.bar() raises TypeError on timedelta dtype
- [x] I have checked that this issue has not already been reported.
- [x] I have confirmed this bug exists on the latest version of pandas.
- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

#### Code Sample, a copy-pastable example

```python
import pandas as pd
df = pd.DataFrame({'ts1': pd.to_datetime([0, 1, 2, 3], unit='s'), 'ts2': pd.to_datetime([1, 1, 5, 4], unit='s')})
diff = df['ts2']-df['ts1']
diff.plot.bar()
```

#### Problem description

Pandas uses `fillna()` to produce valid data points. Unforunately, the data type `timedelta64[ns]` doesn't support setting to integer `0`.

Output:
<details><pre>
TypeError                                 Traceback (most recent call last)
<ipython-input-25-b19437dbe585> in <module>
      2 df = pd.DataFrame({'ts1': pd.to_datetime([0, 1, 2, 3], unit='s'), 'ts2': pd.to_datetime([1, 1, 5, 4], unit='s')})
      3 diff = df['ts2']-df['ts1']
----> 4 diff.plot.bar()

~/.local/lib/python3.9/site-packages/pandas/plotting/_core.py in bar(self, x, y, **kwargs)
   1111         other axis represents a measured value.
   1112         """
-> 1113         return self(kind="bar", x=x, y=y, **kwargs)
   1114 
   1115     @Appender(

~/.local/lib/python3.9/site-packages/pandas/plotting/_core.py in __call__(self, *args, **kwargs)
    953                     data.columns = label_name
    954 
--> 955         return plot_backend.plot(data, kind=kind, **kwargs)
    956 
    957     __call__.__doc__ = __doc__

~/.local/lib/python3.9/site-packages/pandas/plotting/_matplotlib/__init__.py in plot(data, kind, **kwargs)
     59             kwargs["ax"] = getattr(ax, "left_ax", ax)
     60     plot_obj = PLOT_CLASSES[kind](data, **kwargs)
---> 61     plot_obj.generate()
     62     plot_obj.draw()
     63     return plot_obj.result

~/.local/lib/python3.9/site-packages/pandas/plotting/_matplotlib/core.py in generate(self)
    278         self._compute_plot_data()
    279         self._setup_subplots()
--> 280         self._make_plot()
    281         self._add_table()
    282         self._make_legend()

~/.local/lib/python3.9/site-packages/pandas/plotting/_matplotlib/core.py in _make_plot(self)
   1419         K = self.nseries
   1420 
-> 1421         for i, (label, y) in enumerate(self._iter_data(fillna=0)):
   1422             ax = self._get_ax(i)
   1423             kwds = self.kwds.copy()

~/.local/lib/python3.9/site-packages/pandas/plotting/_matplotlib/core.py in _iter_data(self, data, keep_index, fillna)
    256             data = self.data
    257         if fillna is not None:
--> 258             data = data.fillna(fillna)
    259 
    260         for col, values in data.items():

~/.local/lib/python3.9/site-packages/pandas/core/frame.py in fillna(self, value, method, axis, inplace, limit, downcast)
   4457         downcast=None,
   4458     ) -> Optional[DataFrame]:
-> 4459         return super().fillna(
   4460             value=value,
   4461             method=method,

~/.local/lib/python3.9/site-packages/pandas/core/generic.py in fillna(self, value, method, axis, inplace, limit, downcast)
   6424 
   6425             elif not is_list_like(value):
-> 6426                 new_data = self._mgr.fillna(
   6427                     value=value, limit=limit, inplace=inplace, downcast=downcast
   6428                 )

~/.local/lib/python3.9/site-packages/pandas/core/internals/managers.py in fillna(self, value, limit, inplace, downcast)
    619 
    620     def fillna(self, value, limit, inplace: bool, downcast) -> "BlockManager":
--> 621         return self.apply(
    622             "fillna", value=value, limit=limit, inplace=inplace, downcast=downcast
    623         )

~/.local/lib/python3.9/site-packages/pandas/core/internals/managers.py in apply(self, f, align_keys, ignore_failures, **kwargs)
    425                     applied = b.apply(f, **kwargs)
    426                 else:
--> 427                     applied = getattr(b, f)(**kwargs)
    428             except (TypeError, NotImplementedError):
    429                 if not ignore_failures:

~/.local/lib/python3.9/site-packages/pandas/core/internals/blocks.py in fillna(self, value, **kwargs)
   2478         if is_integer(value):
   2479             # Deprecation GH#24694, GH#19233
-> 2480             raise TypeError(
   2481                 "Passing integers to fillna for timedelta64[ns] dtype is no "
   2482                 "longer supported.  To obtain the old behavior, pass "

TypeError: Passing integers to fillna for timedelta64[ns] dtype is no longer supported.  To obtain the old behavior, pass `pd.Timedelta(seconds=n)` instead.
</pre></details>

#### Expected Output

Bar plot.

#### Output of ``pd.show_versions()``

<details><pre>
INSTALLED VERSIONS
------------------
commit           : 9d598a5e1eee26df95b3910e3f2934890d062caa
python           : 3.9.1.final.0
python-bits      : 64
OS               : Linux
OS-release       : 5.10.8-200.fc33.x86_64
Version          : #1 SMP Sun Jan 17 19:53:01 UTC 2021
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : en_US.UTF-8
LANG             : en_US.UTF-8
LOCALE           : en_US.UTF-8

pandas           : 1.2.1
numpy            : 1.19.5
pytz             : 2020.5
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 51.1.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.6.2
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : 0.8.7
xarray           : None
xlrd             : None
xlwt             : None
numba            : None
</pre></details>
