Fix potential PHP error

Author: brandonkelly

src/fields/conditions/RelationalFieldConditionRule.php

@@ -140,7 +140,7 @@ public function modifyQuery(ElementQueryInterface $query): void
             $field instanceof CustomField &&
             $field->getFieldUid() === $this->_fieldUid
         ));
-        if (count($allInstances) > 1) {
+        if ($allInstances && count($allInstances) > 1) {
             $valueSql = $field->getValueSql();
             switch ($this->operator) {
                 case self::OPERATOR_RELATED_TO: