Fix first_valid_index/last_valid_index on DataFrame of all None/NaN values

[cggarvey]

Ensures DataFrame behaves consistent to Series when calling first_valid_index or last_valid_index when all values in df are None/NaN.

• closes df.first_valid_index() fails if there is no valid entry. #17400
• added two test cases in pandas/tests/frame/test_timeseries.py (all other tests of first/last_valid_index() were located there) and verified that they failed prior to/passed after the change.
• passes git diff upstream/master -u -- "*.py" | flake8 --diff

[jreback]

#17488 already has this covered. thanks for the effort. lots of issues out there; welcome other PRs!

[codecov]

Codecov Report

Merging #17646 into master will decrease coverage by 0.02%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master   #17646      +/-   ##
==========================================
- Coverage   91.22%    91.2%   -0.03%     
==========================================
  Files         163      163              
  Lines       49655    49667      +12     
==========================================
+ Hits        45296    45297       +1     
- Misses       4359     4370      +11

Flag	Coverage Δ
#multiple	88.99% <85.71%> (-0.01%)	⬇️
#single	40.16% <0%> (-0.08%)	⬇️

Impacted Files	Coverage Δ
pandas/core/frame.py	97.68% <85.71%> (-0.19%)	⬇️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️

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[codecov]

Codecov Report

Merging #17646 into master will decrease coverage by 0.02%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master   #17646      +/-   ##
==========================================
- Coverage   91.22%    91.2%   -0.03%     
==========================================
  Files         163      163              
  Lines       49655    49667      +12     
==========================================
+ Hits        45296    45297       +1     
- Misses       4359     4370      +11

Flag	Coverage Δ
#multiple	88.99% <85.71%> (-0.01%)	⬇️
#single	40.16% <0%> (-0.08%)	⬇️

Impacted Files	Coverage Δ
pandas/core/frame.py	97.68% <85.71%> (-0.19%)	⬇️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e2757a2...51844c2. Read the comment docs.