Fix Subversion "commit" support in PHP 8.1

Summary:
Premising that "arc commit" is a beautiful Workflow dedicated to svn repositories,
I tried it at work, causing the usual PHP 8.1 deprecation warning:

    $ arc diff
    $ arc commit
    ERROR 8192: preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated at [arcanist/src/workflow/ArcanistWorkflow.php:1520]
    arcanist(head=master, ref.master=e46025f7a914)
      #0 preg_replace(string, string, NULL) called at [<arcanist>/src/workflow/ArcanistWorkflow.php:1520]
      #1 ArcanistWorkflow::normalizeRevisionID(NULL) called at [<arcanist>/src/workflow/ArcanistCommitWorkflow.php:68]
      #2 ArcanistCommitWorkflow::run() called at [<arcanist>/scripts/arcanist.php:427]
    Usage Exception: Unable to identify the revision in the working copy. Use '--revision <revision_id>' to select a revision.

This bug happens at least when Arcanist does not find any related Revision ID.

It seems there is a method that always normalizes the Revision ID, but sometime that is unknown (null).
And so, NULL ends inside a preg_replace(). It's probably OK to have a normalize method that accept wild
things, including NULL. So, fixed that specific method.

Closes T15693

Test Plan:
This revision was tested in production in my company.

Take a random Subversion repository. Edit a line. Run "arc diff". Then run "arc commit". No warnings.

Reviewers: O1 Blessed Committers, aklapper

Reviewed By: O1 Blessed Committers, aklapper

Subscribers: tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15693

Differential Revision: https://we.phorge.it/D25498

src/workflow/ArcanistWorkflow.php

@@ -1516,7 +1516,14 @@ abstract class ArcanistWorkflow extends Phobject {
     }
   }
 
+  /**
+   * @param string|null $revision_id
+   * @return string
+   */
   final protected function normalizeRevisionID($revision_id) {
+    if ($revision_id === null) {
+      return '';
+    }
     return preg_replace('/^D/i', '', $revision_id);
   }