Fix Generate Changelog Action

[afshin]

• Require the branch name
• Fix handling of empty "since" parameter
• Use env naming standards

[blink1073]

Thank you!

[codecov-commenter]

Codecov Report

Merging #126 (5751c65) into master (35213e0) will increase coverage by 0.00%.
The diff coverage is 33.33%.

@@           Coverage Diff           @@
##           master     #126   +/-   ##
=======================================
  Coverage   85.53%   85.53%           
=======================================
  Files          17       17           
  Lines        2101     2102    +1     
  Branches      253      253           
=======================================
+ Hits         1797     1798    +1     
  Misses        213      213           
  Partials       91       91           

Impacted Files	Coverage Δ
jupyter_releaser/actions/generate-changelog.py	0.00% <0.00%> (ø)
jupyter_releaser/changelog.py	88.28% <100.00%> (+0.10%)	⬆️

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