BUG: total_seconds() method returns zero for timedeltas smaller then 1 microsecond
- [ ] I have checked that this issue has not already been reported.

- [ ] 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.

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
import pandas as pd

# create 1us Timedelta - it can be successfully converted to float
a = pd.Timedelta("1us")
print(a)
print(a.total_seconds())

# create 500ns Timedelta - it is not converted successfully to float - result is zero
b = pd.Timedelta("500ns")
print(b)
print(b.total_seconds())

```

#### Problem description
Starting with version 1.1.0 Timedelta objects that represent time shorter than 1 microsecond cannot be successfully converted to float -> the result is always zero. This is also present on the current version (1.2.4). when trying same code on pandas 1.0.5, situation is the opposite:  Printed output for timedeltas smaller then 1us is bad; it shows all zeros, but converted float value is valid. 

Output in pandas 1.0.5:
```
0 days 00:00:00.000001
1e-06
0 days 00:00:00.000000
5e-07
```
third line has unexpected value, but it doesn't affect float conversion, as can be seen in the 4th line.

Output in pandas 1.2.4:
```
0 days 00:00:00.000001
1e-06
0 days 00:00:00.000000500
0.0
```
now the timedelta representation is fixed, but its float value is 0.0 insteaad of 5e-7

#### Expected Output
```
0 days 00:00:00.000001
1e-06
0 days 00:00:00.000000500
5e-7
```

I am also interested in contributing and helping to fix this issue, if that is needed.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 2cb96529396d93b46abab7bbc73a208e708c642e
python           : 3.8.4.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.19041
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : Serbian (Latin)_Serbia.1250

pandas           : 1.2.4
numpy            : 1.20.2
pytz             : 2021.1
dateutil         : 2.8.1
pip              : 20.1.1
setuptools       : 47.1.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : None
IPython          : None
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : None
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pyxlsb           : None
s3fs             : None
scipy            : None
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None

</details>
