Comment template functions don't check for comment existence

[SergeyBiryukov]

Discovered during debugging an issue with Circle Lite theme, which uses ​comment_text( true ) in kopa_comment_callback() function.

That, while being an invalid usage (the function treats true as a comment ID), causes an inconsistent behaviour, depending on the existence of the comment:

• If the comment with ID 1 exists, the function returns its text, as expected. That's obviously not what the author of the theme intended, but still the correct behaviour of the function.
• If the comment with ID 1 does not exist, the function returns the text of the current comment in the loop instead of an empty string. What happens is get_comment( $comment_ID ) returns null (as expected), but then null is passed to get_comment_text(), which treats it as the current comment.

After investigating more, it looks like most of comment template functions are either affected in a similar way or cause an undefined property notice when passed a non-existing comment ID. Let's bring some consistency here.

[SergeyBiryukov]

40143.patch​ fixes the inconsistencies and adds a couple of tests, but needs more tests.

All existing comment tests pass.

[flixos90]

Due to lack of feedback and discussion, punting this to future release. Feel free to put it back into the milestone if this is actually ready.