The placement rule may leave the empty region can not be merged

## Enhancement Task
Consider the following case: We have three tables: A, B, and C. Both tables A and C have a placement rule. Table B is empty, but it cannot be merged to either A or C. Since the A and C need to meet the rule boundary. When this situation happens, there might be a lot of regions in our monitor which is very confused. We need to improve it or provide a way to reduce user concerns.