feat: Allow empty cells in typed tables

[mgax]

Fix #282.

⚠️ Migrations will conflict with #388. Whichever gets merged first, the migrations in the other branch should just be deleted and re-created, they are just a side-effect of the Streamfield schema being changed.

By setting the content blocks as required=False, empty values are allowed. They are displayed as blank cells, even the "Numeric" (FloatBlock) which would, by default, render as None.

In the page editor:

Rendered on the page:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.15%. Comparing base (4ad3143) to head (df8de30).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #403      +/-   ##
==========================================
+ Coverage   82.72%   83.15%   +0.42%     
==========================================
  Files         124      130       +6     
  Lines        2599     2665      +66     
==========================================
+ Hits         2150     2216      +66     
  Misses        449      449              

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[microamp]

While I'm not experienced in generating migration scripts in Django/Wagtail, setting required=False for these fields does make sense to me.