fix: small fix when wrong placeholder id is required

editor/filters/api.php

@@ -46,7 +46,10 @@ public function filterPlaceholdersContent() {
 				 */
 				$placeholder = $context->getPlaceholderById($placeholderId);
 
-				if(!$placeholder) continue;
+				if(!$placeholder) {
+                    $placeholderContents[$placeholderId] = '';
+                    continue;
+                }
 
 				/**
 				 * @var Brizy_Content_Placeholders_Abstract $placeholderInstance;