DeployFormsBug7530: Do not pass null values to strlen (#2977)

Since PHP 8.0 passing of null to strlen is deprecated and hence must be
avoided.

Author: lippserd

application/forms/DeployFormsBug7530.php

@@ -13,7 +13,7 @@ public function hasBeenSubmitted()
         if (parent::hasBeenSubmitted()) {
             return true;
         } else {
-            return \strlen($this->getSentValue('confirm_7530')) > 0;
+            return strlen($this->getSentValue('confirm_7530', '')) > 0;
         }
     }