REST API: Add tests for empty or "no-op" updates

[jnylen0]

"Empty" updates that don't change any stored values should be allowed for all resource types. This patch adds tests accordingly for posts, terms, and users.

This is currently broken for comments. See ticket:38700#comment:14 for details.

[joehoyle]

Given we also have the test for comments in #38700, maybe we should remove it from this patch, and then we can commit.

[jnylen0]

In 39371:

REST API: Add tests for empty or "no-op" updates.

The API should allow updates that don't actually change anything. This allows
clients to, for example, accidentally send the same request twice without
encountering unexpected errors. This currently works for posts, terms, and
users, so this commit adds test cases accordingly.

See #38700 for issues preventing this from working for comments.

Fixes #38975.

[pento]

In 39372:

REST API: Add tests for empty or "no-op" updates.

The API should allow updates that don't actually change anything. This allows clients to, for example, accidentally send the same request twice without encountering unexpected errors. This currently works for posts, terms, and users, so this commit adds test cases accordingly.

See #38700 for issues preventing this from working for comments.

Merge of [39371] to the 4.7 branch.

Props jnylen0.
See #38975.