More cleanup of tag lock

[afshin]

Follow up to #93 to handle the case where there are no tags on the branch.

[codecov-commenter]

Codecov Report

Merging #95 (55f97a8) into master (cedc3c6) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master      #95   +/-   ##
=======================================
  Coverage   85.03%   85.03%           
=======================================
  Files          16       16           
  Lines        1925     1925           
  Branches      242      242           
=======================================
  Hits         1637     1637           
  Misses        196      196           
  Partials       92       92           

Impacted Files	Coverage Δ
jupyter_releaser/actions/draft_release.py	0.00% <0.00%> (ø)
jupyter_releaser/__init__.py	100.00% <100.00%> (ø)

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[blink1073]

Thank you!