Editable: Split caption if empty

[youknowriad]

Fixes #2173

This PR adds a new PR to the Editable component splitIfEmpty which causes the Editable to call onSplit only if it's empty. I'm not totally satisfied but can't think of a better alternative.

This is only implemented for image captions, could be generalized to other captions if we agree on the approach.

[codecov]

Codecov Report

Merging #2664 into master will decrease coverage by 0.04%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2664      +/-   ##
==========================================
- Coverage   31.31%   31.26%   -0.05%     
==========================================
  Files         178      178              
  Lines        5429     5437       +8     
  Branches      949      952       +3     
==========================================
  Hits         1700     1700              
- Misses       3154     3159       +5     
- Partials      575      578       +3

Impacted Files	Coverage Δ
blocks/editable/index.js	10.27% <0%> (-0.25%)	⬇️
blocks/library/image/block.js	0% <0%> (ø)	⬆️

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[aduth]

Do we even care to support image captions with line breaks?

This isn't precisely what was requested in #2173, where the behavior was suggested to be "move to next focusable", not inserting a new block.

I'll be the dissenting opinion that splitting behavior by whether content exists is awkward and unpredictable. Personally I'd rather we have it so that if the Editable is not explicitly multiline supporting, Enter can proceed to the next focusable, and does so in the WritingFlow component. But making this opt-in or block-specific seems asking for trouble.

[youknowriad]

Closing as too complex for this purpose, a more generic solution in WritingFlow could be better as suggested by @aduth.

Also, this is less of an issue since we have arrow navigation.