fix: resolve failure in _increment_stream_state() for cases when replication_method is LOG_BASED

[aaronsteers]

I did not add any extra tests.

Would be good to pick those up and add official handling for LOG_BASED incremental at the same time.

Reviewing with whitespace changes hidden will help to highlight changed lines.

While creating this PR, I remembered that get_context_state() is not a no-op, so I added a comment to help future maintainers.

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📚 Documentation preview 📚: https://meltano-sdk--1126.org.readthedocs.build/en/1126/

[codecov]

Codecov Report

Merging #1126 (4f98e29) into main (81ec6ac) will increase coverage by 0.02%.
The diff coverage is 42.85%.

❗ Current head 4f98e29 differs from pull request most recent head 2310063. Consider uploading reports for the commit 2310063 to get more accurate results

@@            Coverage Diff             @@
##             main    #1126      +/-   ##
==========================================
+ Coverage   83.47%   83.49%   +0.02%     
==========================================
  Files          42       42              
  Lines        3855     3854       -1     
  Branches      658      657       -1     
==========================================
  Hits         3218     3218              
  Misses        472      472              
+ Partials      165      164       -1     

Impacted Files	Coverage Δ
singer_sdk/streams/core.py	85.50% <42.85%> (+0.20%)	⬆️

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[aaronsteers]

Codecov CI failure is basically same as before; no increase or decrease per se.