Fix ModelCheckpoint callback for no loggers case

[ioangatop]

What does this PR do?

Fixes #18865

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📚 Documentation preview 📚: https://pytorch-lightning--18867.org.readthedocs.build/en/18867/

[ioangatop]

I've made a minimal quick fix - probably the most appropriate will be to make a setter and a getter for the self.logdir.

Whatever you prefer but also it could be in a different, follow up PR.

[codecov]

Codecov Report

Merging #18867 (2157272) into master (0843041) will decrease coverage by 27%.
The diff coverage is 100%.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #18867     +/-   ##
=========================================
- Coverage      75%      48%    -27%     
=========================================
  Files         443      435      -8     
  Lines       35448    35296    -152     
=========================================
- Hits        26718    16926   -9792     
- Misses       8730    18370   +9640     

[carmocca]

Issue description makes sense! Also to make this a setter in a follow-up. Thank you!