Add warning if IIASA API returns empty

[danielhuppmann]

Please confirm that this PR has done the following:

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

Description of PR

This PR fixes an unexpected error when querying data via the IIASA API from a database that is empty. It also adds the option to filter index(), meta() and properties().

closes #676

[codecov[bot]]

Codecov Report

Merging #697 (6998744) into main (7f00c79) will increase coverage by 0.0%. The diff coverage is 95.5%.

@@          Coverage Diff          @@
##            main    #697   +/-   ##
=====================================
  Coverage   94.8%   94.8%           
=====================================
  Files         58      58           
  Lines       5823    5856   +33     
=====================================
+ Hits        5525    5557   +32     
- Misses       298     299    +1     

Impacted Files	Coverage Δ
pyam/iiasa.py	86.1% <92.3%> (+0.1%)	:arrow_up:
tests/test_iiasa.py	97.5% <100.0%> (+0.3%)	:arrow_up:

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