Fix bin/fill-locales when the key in en is a hash and not in other

[albertchae]

The script would fail if the current en was a hash but other wasn't
because we would try to call dig on a string. In this case, return
nil so that we can recursively copy the shape of en to other

I'm not sure if this is the best way to fix this, but I did use it to copy over the keys in #4466 and also tested that it works on #4462

[simi]

Looks good! Thanks for the fix. This is much better than crash and the problematic case is IMHO rare, no need to do anything complex.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a166302) 97.13% compared to head (f342545) 97.15%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4467      +/-   ##
==========================================
+ Coverage   97.13%   97.15%   +0.02%     
==========================================
  Files         385      391       +6     
  Lines        8200     8259      +59     
==========================================
+ Hits         7965     8024      +59     
  Misses        235      235              

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