REST API: Allow sending an empty or no-op comment update.

In general, updates that don't actually change anything should succeed.
[39371] added tests for other object types, and this commit fixes empty updates
for comments and adds the missing test.

Fixes #38700.

Built from https://develop.svn.wordpress.org/trunk@39597


git-svn-id: https://core.svn.wordpress.org/trunk@39537 1a063a9b-81f0-0310-95a4-ce76da25c4cd