Fix error in user search

The error is caused by an empty radiobutton.
```
preg_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated
```

wcfsetup/install/files/lib/system/option/RadioButtonOptionType.class.php

@@ -105,7 +105,7 @@ public function getCondition(PreparedStatementConditionBuilder &$conditions, Opt
             return false;
         }
 
-        $conditions->add("option_value.userOption" . $option->optionID . " = ?", [StringUtil::trim($value)]);
+        $conditions->add("option_value.userOption" . $option->optionID . " = ?", [StringUtil::trim($value ?: '')]);
 
         return true;
     }