Enable renaming of an empty IamDataFrame

[danielhuppmann]

Please confirm that this PR has done the following:

• [x] Tests Added
• ~Documentation Added~
• ~Name of contributors Added to AUTHORS.rst~
• ~Description in RELEASE_NOTES.md Added~

Description of PR

This PR fixes an unexpected error when using rename() on an empty IamDataFrame.

Related to https://github.com/IAMconsortium/nomenclature/pull/123

closes #631

[codecov[bot]]

Codecov Report

Merging #668 (26dc7ae) into main (de6b47b) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #668   +/-   ##
=====================================
  Coverage   94.6%   94.6%           
=====================================
  Files         59      59           
  Lines       5770    5776    +6     
=====================================
+ Hits        5459    5465    +6     
  Misses       311     311           

Impacted Files	Coverage Δ
pyam/core.py	94.6% <100.0%> (+<0.1%)	:arrow_up:
tests/test_feature_rename.py	100.0% <100.0%> (ø)

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[phackstock]

It looks like it failing at the point of using actions/setup-python@v2, I quickly checked and there is a version 3 available of setup-python (https://github.com/marketplace/actions/setup-python). Maybe upgrading to this one would fix the problem?

[danielhuppmann]

Thanks for the assist, @phackstock!

[phackstock]

Fingers crossed that that fixes it.

[phackstock]

It worked :tada: