Handle the case where there are no changes in changelog

[blink1073]

Handle the case where there are no changes to the changelog, which can happen when making an automated changelog pull request. Fixes error seen in https://github.com/jupyter-server/jupyter_releaser/actions/runs/1005457462.

[codecov-commenter]

Codecov Report

Merging #61 (38751f3) into master (5b17742) will decrease coverage by 1.16%.
The diff coverage is 76.62%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   89.03%   87.87%   -1.17%     
==========================================
  Files          15       15              
  Lines        1697     1757      +60     
  Branches      198      213      +15     
==========================================
+ Hits         1511     1544      +33     
- Misses        118      133      +15     
- Partials       68       80      +12     

Impacted Files	Coverage Δ
jupyter_releaser/tests/test_functions.py	100.00% <ø> (ø)
jupyter_releaser/util.py	82.19% <37.50%> (-2.71%)	⬇️
jupyter_releaser/changelog.py	88.18% <62.50%> (-2.56%)	⬇️
jupyter_releaser/lib.py	74.92% <69.44%> (-2.82%)	⬇️
jupyter_releaser/npm.py	80.14% <70.37%> (-3.60%)	⬇️
jupyter_releaser/tests/test_cli.py	97.64% <91.66%> (-0.20%)	⬇️
jupyter_releaser/__init__.py	100.00% <100.00%> (ø)
jupyter_releaser/cli.py	92.37% <100.00%> (+0.13%)	⬆️
jupyter_releaser/python.py	91.66% <100.00%> (+3.78%)	⬆️
jupyter_releaser/tests/conftest.py	98.50% <100.00%> (+0.01%)	⬆️

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