short circuit out if CommentContent is not defined

In certain circumstance tickets are created (via FormTools or whatever)
where there is no CommentOnCreate box. Thus, the argument
'CommentContent' doesn't even exist. This patch fixes warnings like the
following:

[warning]: Use of uninitialized value in pattern match (m//) at
RT-Extension-CommentOnCreate/html/Callbacks/RT-Extension-CommentOnCreate/Ticket/Display.html/BeforeDisplay
line 49.