REST API: Updating a comment without sending content is valid, but unsupported

[jnylen0]

The REST API currently prohibits updating a comment unless the content is specified along with the update. Originally introduced ​here (and then shuffled around a bit afterwards).

This doesn't make sense with the rest of the API: it's possible (and common) to update only a couple of attributes at a time for a post.

Example use cases:

• Fixing a typo in the author name
• Resetting the parent of a comment if it was placed incorrectly on a p2

[jnylen0]

We still want to prohibit comment updates with content: ''.

[jnylen0]

Let's also not change the error responses for no reason.

[jnylen0]

Aaaand... let's not add unnecessary tests either.

[joehoyle]

In 39196:

REST API: Allow updating a comment without the content present.

For all resources in the REST API, sending partial updates is supported. This fixes needing to _always_ specify comment content.

Props jnylen.
Fixes #38720.