Fix tzawareness_compat for DatetimeIndex comparisons with NaT

[jbrockmendel]

ATM pd.date_range('2016-01-01', periods=1, tz='US/Pacific') < pd.NaT raises TypeError because NaT is tz-naive. This fixes that.

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #19276 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #19276      +/-   ##
==========================================
- Coverage   91.56%   91.53%   -0.03%     
==========================================
  Files         148      148              
  Lines       48874    48876       +2     
==========================================
- Hits        44751    44741      -10     
- Misses       4123     4135      +12

Flag	Coverage Δ
#multiple	89.91% <100%> (-0.03%)	⬇️
#single	41.69% <66.66%> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/indexes/datetimes.py	95.24% <100%> (ø)	⬆️
pandas/plotting/_converter.py	65.22% <0%> (-1.74%)	⬇️

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[jreback]

thanks!