Fix Pholio RuntimeException: Undefined variable $dictionary (when adding an empty Inline Comment)

Summary:
Trying to create an empty inline comment in a Pholio mock, $dictionary does not get set as both strlen($v_content) and $inline->getID() are not true.

Thus show a more explanatory error message ('Comment cannot be empty.') to users instead of exposing internal variable names.

EXCEPTION: (RuntimeException) Undefined variable $dictionary at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=b325304b6e52), phorge(head=master, ref.master=980293b707a0)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/applications/pholio/controller/PholioInlineController.php:117]

Closes T15456

Test Plan: After applying this change, try to add an empty Inline Comment in a Pholio mock. See that the error message is now "Comment cannot be empty." instead of "Undefined variable $dictionary".

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15456

Differential Revision: https://we.phorge.it/D25281