Allow unsuspending from timed suspension

Fixes flarum/core#1345.

extensions/suspend/src/Listener/SaveSuspensionToDatabase.php

@@ -59,7 +59,9 @@ class SaveSuspensionToDatabase
 
             $this->assertCan($actor, 'suspend', $user);
 
-            $user->suspend_until = new DateTime($attributes['suspendUntil']);
+            $user->suspend_until = $attributes['suspendUntil']
+                ? new DateTime($attributes['suspendUntil'])
+                : null;
         }
     }
 }

extensions/suspend/src/SuspendValidator.php

@@ -19,6 +19,6 @@ class SuspendValidator extends AbstractValidator
      * {@inheritdoc}
      */
     protected $rules = [
-        'suspendUntil' => ['date'],
+        'suspendUntil' => ['nullable', 'date'],
     ];
 }