64049: REST API: Allow comments to be edited to become empty

[kadamwhite]

Originally proposed by @adamsilverstein in #9988, this PR adds tests to his fix while also resolving a potential logical error which could prevent updates to unrelated comment object properties.

• Enable empty block comments #9988 as written causes an error in some existing unit tests (including my newly-added one for this ticket), Undefined array key "comment_content" (failing on the last line of check_is_comment_content_allowed) because the controller expects to be able to process partial objects: For example, you can pass an object in a PUT which changes the author but alters no other fields.
• We therefore only want to check the content for emptiness (and verify using the filter whether that is allowable) when the property has been passed back.
• Currently, the only path through which check_is_comment_content_allowed() can be called is during the create_item method, which ensures comment_content is initialized to '' if not present — this should not result in any change to existing behavior on comment creation or updates.

Trac ticket: https://core.trac.wordpress.org/ticket/64049

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[mukeshpanchal27]

The inline doc needs to changed now it return 200 response.