Bugfix for non-empty invisible header in 'meta' sheet

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added ->bugfix, no functional change
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR resolves an unexpected error reported by @fbenke-pik when initializing an IamDataFrame from an xlsx file where the meta sheet has only a header row and also a has non-empty but invisible entry in that header row. (yes, this is a corner case)

[codecov]

Codecov Report

Merging #508 (4b9681e) into main (956d1d6) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #508   +/-   ##
=====================================
  Coverage   93.0%   93.0%           
=====================================
  Files         43      43           
  Lines       4867    4876    +9     
=====================================
+ Hits        4528    4537    +9     
  Misses       339     339           

Impacted Files	Coverage Δ
pyam/utils.py	91.9% <100.0%> (+0.1%)	⬆️
tests/test_io.py	100.0% <100.0%> (ø)

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[fbenke-pik]

Looks good to me!

[danielhuppmann]

Resolving data import corner cases one PR at a time... 😜