Merge pull request #3968 from nextcloud/backport/3967/stable30

[stable30] fix: Catch parsing errors or null doc structure

lib/Service/TemplateFieldService.php

@@ -86,7 +86,7 @@ public function extractFields(Node|int $file): array {
 				$form
 			);
 
-			$documentStructure = json_decode($response->getBody(), true)['DocStructure'];
+			$documentStructure = json_decode($response->getBody(), true)['DocStructure'] ?? [];
 			$fields = [];
 
 			foreach ($documentStructure as $index => $attr) {