Handle text field clearing when syncing

[zerolab]

Fixes #467

tl;dr - we mark text fields and StreamFields as TranslateableField, but only flag StreamFields as needing syncing. When the value is removed, there is nothing to tell Translation.create_or_update_translation() that this has happened.

Translation._get_segments_for_translation() only return translateable fields that have values in them, so nothing will happen via ingest_segments

[codecov-commenter]

Codecov Report

Merging #495 (eb950c9) into main (297e5ed) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #495   +/-   ##
=======================================
  Coverage   92.82%   92.83%           
=======================================
  Files          36       36           
  Lines        2983     2985    +2     
  Branches      467      468    +1     
=======================================
+ Hits         2769     2771    +2     
  Misses        114      114           
  Partials      100      100           

Impacted Files	Coverage Δ
wagtail_localize/fields.py	91.07% <100.00%> (+0.16%)	⬆️

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[chris48s]

I don't think my 👍 counts for anything on this repo but LGTM