Fix a PHP 8.1 deprecated use of preg_match with a NULL argument

Summary:
These calls are preventing users to browse subversion/mercurial repositories in PHP 8.1+.
Indeed, a similar bug affecting git repositories was already addressed in another commit
(rP6b8ec50148909938850b5acfd11725ae23a8e31b). This commit harmonize both DiffusionSvnRequest
and DiffusionMercurialRequest with DiffusionGitRequest

Fix T15607

Test Plan:
- Sign in
- Open a diffusion SVN/Mercurial repository
- You should not get a RuntimeException concerning this preg_match call

Reviewers: O1 Blessed Committers, Sten, valerio.bozzolan

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

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

Maniphest Tasks: T15607

Differential Revision: https://we.phorge.it/D25397

src/applications/diffusion/request/DiffusionMercurialRequest.php

@@ -3,6 +3,9 @@
 final class DiffusionMercurialRequest extends DiffusionRequest {
 
   protected function isStableCommit($symbol) {
+    if ($symbol === null) {
+      return false;
+    }
     return preg_match('/^[a-f0-9]{40}\z/', $symbol);
   }
 
@@ -10,11 +13,9 @@ final class DiffusionMercurialRequest extends DiffusionRequest {
     if ($this->branch) {
       return $this->branch;
     }
-
     if ($this->repository) {
       return $this->repository->getDefaultBranch();
     }
-
     throw new Exception(pht('Unable to determine branch!'));
   }
 

src/applications/diffusion/request/DiffusionSvnRequest.php

@@ -3,6 +3,9 @@
 final class DiffusionSvnRequest extends DiffusionRequest {
 
   protected function isStableCommit($symbol) {
+    if ($symbol === null) {
+      return false;
+    }
     return preg_match('/^[1-9]\d*\z/', $symbol);
   }