BUG: Fix pd.Timedelta(None) to return NaT.
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Before this commit pd.Timedelta raised ValueError
when None is passed.
This behavior is inconsistent because
- pd.Timestamp(None) and pd.Period(None) return NaT
- pd.Timedelta returns NaT if '', 'nat', 'NAT' or np.nan
  is passed

This PR is related to https://github.com/pydata/pandas/pull/13687#issuecomment-233204955
