PHP 8.1 "preg_match(null)" exception in javelin/markup.php when http_action not set

Summary:
Accessing a project's workboard URL of a non-existing workboard shows RunTimeException in PHP 8.1:

preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated

Closes T15262

Test Plan: Page whether to create a workboard for a project was displayed after this change

Reviewers: O1 Blessed Committers, avivey, valerio.bozzolan

Reviewed By: O1 Blessed Committers, avivey, valerio.bozzolan

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

Maniphest Tasks: T15262

Differential Revision: https://we.phorge.it/D25131