implemented test to prevent blank tags

frontend/src/routes/tags/TagList.svelte

@@ -17,6 +17,7 @@
     let router = useTinyRouter();
 
     async function addTag(){
+        if (!newTag) return;
         if (!id) {
             // when creating elements, they don`t have an id, yet
             tags.push(newTag);

tags/src/main/java/de/srsoftware/umbrella/tags/TagModule.java

@@ -93,7 +93,7 @@ public class TagModule extends BaseHandler implements TagService {
 			var head = path.pop();
 			long entityId = Long.parseLong(head);
 			var json = json(ex);
-			if (!(json.has(TAG) && json.get(TAG) instanceof String tag)) throw missingFieldException(TAG);
+			if (!(json.has(TAG) && json.get(TAG) instanceof String tag && !tag.isBlank())) throw missingFieldException(TAG);
 			List<Long> userList = null;
 			if (!json.has(USER_LIST)) throw missingFieldException(USER_LIST);
 			var ul = json.isNull(USER_LIST) ? null : json.get(USER_LIST);