Avoid empty markup in comments

[obenland]

When comment_reply_link() doesn't display a link, like when comments are closed, both Walker_Comment::comment() and Walker_Comment::html5_comment() still display the empty wrapper div.

This can lead to design irregularities when margins or paddings are applied to that element, but shouldn't since the link to pad is non-existent.

[obenland]

Tagging with twenty-fifteen to keep track of it.

[anonymized_8769252]

Tested the patch, looks good to me.

[ocean90]

In 29908:

Comments: Don't print an empty HTML markup when comment_reply_link() returns no link.

props obenland.
fixes #29895.

[helen]

#25203 was marked as a duplicate.