Add tests of add_par() with empty DataFrames

[khaeru]

How to review

Note that CI checks pass, in particular newly added tests/parametrizations of test_add_par and test_add_par2.

PR checklist

• [x] Add or expand tests.
  • [x] Test of an empty dataframe with correct columns.
  • [x] Test of an empty dataframe for a parameter with different idx_names/idx_sets, per https://github.com/iiasa/ixmp/issues/373#issuecomment-712056459
• ~Add, expand, or update documentation.~ N/A, bug fix
• [x] Update release notes.

[codecov[bot]]

Codecov Report

Merging #374 (df500f8) into master (ed3499f) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff           @@
##           master    #374   +/-   ##
======================================
  Coverage    96.8%   96.8%           
======================================
  Files          46      46           
  Lines        5324    5334   +10     
======================================
+ Hits         5157    5168   +11     
+ Misses        167     166    -1     

Impacted Files	Coverage Δ
ixmp/core.py	95.1% <100.0%> (+0.2%)	:arrow_up:
ixmp/tests/core/test_scenario.py	100.0% <100.0%> (ø)