Fixed empty string replacement bug

[coordt]

Only a missing replacement value will trigger one of the fallback options.

Fixes #117

[github-actions]

Version hint: patch
Current version: 0.16.0
New version (when merged): 0.16.1

_{Comment ID: Display the version hint-auto-generated}

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (8db9552) 92.90% compared to head (d9965ab) 92.84%.
Report is 2 commits behind head on master.

Files	Patch %	Lines
bumpversion/files.py	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   92.90%   92.84%   -0.07%     
==========================================
  Files          19       19              
  Lines        1255     1257       +2     
  Branches      259      260       +1     
==========================================
+ Hits         1166     1167       +1     
  Misses         60       60              
- Partials       29       30       +1     

Flag	Coverage Δ
python-3.10	92.84% <66.66%> (-0.07%)	⬇️
python-3.11	92.84% <66.66%> (-0.07%)	⬇️
python-3.8	92.84% <66.66%> (-0.07%)	⬇️
python-3.9	92.84% <66.66%> (-0.27%)	⬇️

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