Fix TableTraits functions failing on non-standard index

[ReubenJ]

When using an index other than the default RangeIndex, the TableTraits functions currently fail as they assume a monotonically increasing range beginning from 0.

This PR addresses that issue.

I also cleaned up the code as it was adding an extra special case for Strings that was not necessary.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (main@9d20bc6). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #103   +/-   ##
=======================================
  Coverage        ?   63.46%           
=======================================
  Files           ?        5           
  Lines           ?      260           
  Branches        ?        0           
=======================================
  Hits            ?      165           
  Misses          ?       95           
  Partials        ?        0           

Flag	Coverage Δ
unittests	63.46% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.