Small internal cleanups

[dbussink]

This cleans up some logic around the default collation and coercion. We now make the MySQL 8 collations the default if there's no version mismatch instead of what we had with MySQL 5.7.

Additionally we fix a small issue that doesn't really matter in practice with an unknown collation, but we know have a full type so we do know it, so let's use it.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.65%. Comparing base (ae7214d) to head (27cd8f9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16467   +/-   ##
=======================================
  Coverage   68.64%   68.65%           
=======================================
  Files        1549     1549           
  Lines      199214   199212    -2     
=======================================
+ Hits       136750   136762   +12     
+ Misses      62464    62450   -14     

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