BUG: Series/DataFrame with IntervalIndex doesn't display NaN in index

[simonjayhawkins]

• closes BUG: Series/DataFrame with IntervalIndex doesn't display NaN in index  #25984
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #25993 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #25993      +/-   ##
==========================================
- Coverage   91.86%   91.85%   -0.01%     
==========================================
  Files         175      175              
  Lines       52547    52547              
==========================================
- Hits        48271    48266       -5     
- Misses       4276     4281       +5

Flag	Coverage Δ
#multiple	90.41% <100%> (ø)	⬆️
#single	41.9% <100%> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/core/indexes/interval.py	95.25% <100%> (ø)	⬆️
pandas/io/gbq.py	75% <0%> (-12.5%)	⬇️
pandas/core/frame.py	96.79% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.62% <0%> (-0.11%)	⬇️

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[jreback]

lgtm. i restarted the build, ping on green.

[simonjayhawkins]

green

[jreback]

thanks @simonjayhawkins