Comments: Ensure that inserting a comment with an empty type results in correct `comment` type.

Add unit tests for `wp_handle_comment_submission()` and `wp_insert_comment()` receiving an empty type.

Follow-up to [47597].

Props ocean90, imath.
Fixes #49236.
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git-svn-id: https://core.svn.wordpress.org/trunk@47401 1a063a9b-81f0-0310-95a4-ce76da25c4cd