fix: prevent prerelase from creating a bump when there are no commits

[woile]

Closes #281

Checklist

• Add test cases to all the changes you introduce
• Run ./script/format and ./script/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually

Steps to Test This Pull Request

Described in the issue

Additional context

This issue can only be triggered if there are commits but they are not elegible for an increment.

• let's say tag exists 0.1.0
• a new commit is created "ci: update job name"
• cz bumo -pr alpha is executed
• commitizen will find commits, but no increment, thus triggering the bug that happens afterwards.

[codecov]

Codecov Report

Merging #307 (7bafa42) into master (720befe) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #307   +/-   ##
=======================================
  Coverage   96.96%   96.97%           
=======================================
  Files          33       33           
  Lines         924      926    +2     
=======================================
+ Hits          896      898    +2     
  Misses         28       28           

Flag	Coverage Δ
unittests	96.97% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
commitizen/__version__.py	100.00% <100.00%> (ø)
commitizen/commands/bump.py	94.44% <100.00%> (+0.12%)	⬆️

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[woile]

potential issue:

• can an alpha version become a beta version with no commits? if yes, then this RP breaks that behavior

solved:
this check is only triggered when going from normal release to a pre-release, not from pre-release to pre-release.
it also provides a way to fix the issue: you can transition to pre-release with no commits, but you have to manually specify the increment using --increment.