[2.x] Change default DocumentationPage group to be null instead of 'other'

[emmadesilva]

It's kinda weird that we've been using 'other' as a sort of pseudo-null state. It only makes sense for the group to be other when there are other groups, so that should be handled in the frontend Blade, thus we instead now null coalesce to 'Other' when creating the group title.

This is breaking and targets V2 (specifically #1568)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4dca8c5) 100.00% compared to head (4be1a45) 100.00%.

❗ Current head 4be1a45 differs from pull request most recent head 3ce0967. Consider uploading reports for the commit 3ce0967 to get more accurate results

Additional details and impacted files

@@                        Coverage Diff                        @@
##             improved-navigation-internals     #1572   +/-   ##
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  Coverage                           100.00%   100.00%           
+ Complexity                            1761      1760    -1     
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  Files                                  184       184           
  Lines                                 4765      4765           
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  Hits                                  4765      4765           

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