[Minor] Optionals should not be set to null (apache#6276)

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

Optionals should not be set to null

### Why are the changes needed?

see above

Fix: # N/A

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

No

### How was this patch tested?

Tested locally

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

@@ -68,7 +68,7 @@ public OwnerDetails(
   /** Displays the owner of an entity. */
   @Override
   public void handle() {
-    Optional<Owner> owner = null;
+    Optional<Owner> owner = Optional.empty();
     MetadataObject metadata = MetadataObjects.parse(entity, entityType);
 
     try {