BUG: Timedelta input string with only symbols and no digits failed to raise an error
- [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.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

---

`pd.Timedelta` with non-digit string input, such as `pd.Timedelta('-')`, successfully processes and returns `Timedelta('0 days 00:00:00')`. From discussion with @jreback in #39497 it was concluded that this is unwanted and that it should raise an error instead.

I found that this appears for string input with `'+'`, `'-'`, `','` and `' '` in various combinations, see below

```python
In [1]: pd.Timedelta('+')
Out[1]: Timedelta('0 days 00:00:00')

In [2]: pd.Timedelta('-')
Out[2]: Timedelta('0 days 00:00:00')

In [3]: pd.Timedelta(' ')
Out[3]: Timedelta('0 days 00:00:00')

In [4]: pd.Timedelta(',')
Out[4]: Timedelta('0 days 00:00:00')

In [5]: pd.Timedelta('++')
Out[5]: Timedelta('0 days 00:00:00')

In [6]: pd.Timedelta('+-')
Out[6]: Timedelta('0 days 00:00:00')
```

#### Problem description

Invalid input should raise an error instead of successfully resolving to 0

#### Expected Output

Raise an error. I would propose to generalize the `ValueError("unit abbreviation w/o a number")` on [L497](https://github.com/pandas-dev/pandas/blob/master/pandas/_libs/tslibs/timedeltas.pyx#L497) of `timedeltas.pyx` to `ValueError("characters w/o a number")` . Since  this error also shows up if there are no unit abbrevations, for example:

```python
In [2]: pd.Timedelta('foo')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-2-06e16138b2f6> in <module>
----> 1 pd.Timedelta('foo')

~/Documents/developer/pandas/pandas/_libs/tslibs/timedeltas.pyx in pandas._libs.tslibs.timedeltas.Timedelta.__new__()
   1192                 value = parse_iso_format_string(value)
   1193             else:
-> 1194                 value = parse_timedelta_string(value)
   1195             value = np.timedelta64(value)
   1196         elif PyDelta_Check(value):

~/Documents/developer/pandas/pandas/_libs/tslibs/timedeltas.pyx in pandas._libs.tslibs.timedeltas.parse_timedelta_string()
    429             result += timedelta_as_neg(r, neg)
    430         else:
--> 431             raise ValueError("unit abbreviation w/o a number")
    432
    433     # treat as nanoseconds

ValueError: unit abbreviation w/o a number
```

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

<details>

INSTALLED VERSIONS
------------------
commit           : 492c5e00d112f0977a24d219f3758c6757fa2f7c
python           : 3.8.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 20.2.0
Version          : Darwin Kernel Version 20.2.0: Wed Dec  2 20:39:59 PST 2020; root:xnu-7195.60.75~1/RELEASE_X86_64
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.UTF-8

pandas           : 1.2.0.dev0+1259.g492c5e00d1
numpy            : 1.19.2
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.4
setuptools       : 49.6.0.post20201009
Cython           : 0.29.21
pytest           : 6.1.1
hypothesis       : 5.37.3
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : 1.3.7
lxml.etree       : 4.6.0
html5lib         : 1.1
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.3.2
fsspec           : 0.8.4
fastparquet      : 0.4.1
gcsfs            : 0.7.1
matplotlib       : 3.3.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : 1.0.1
pyxlsb           : None
s3fs             : 0.4.2
scipy            : 1.5.2
sqlalchemy       : 1.3.20
tables           : 3.6.1
tabulate         : 0.8.7
xarray           : 0.16.1
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.51.2

</details>
