[#5930] improvement(CLI): improve unknown tag output. (#5978)

### What changes were proposed in this pull request?

Improve output when CLI add an unknown tags.

### Why are the changes needed?

Fix: #5930 

### Does this PR introduce _any_ user-facing change?

NO

### How was this patch tested?

local test

clients/cli/src/main/java/org/apache/gravitino/cli/commands/TagEntity.java

@@ -98,7 +98,7 @@ public void handle() {
       exitWithError(exp.getMessage());
     }
 
-    String all = String.join(",", tagsToAdd);
+    String all = tagsToAdd.length == 0 ? "nothing" : String.join(",", tagsToAdd);
 
     System.out.println(entity + " now tagged with " + all);
   }