Improve task range completion prerequisite check

[yycptt]

What changed?

• A follow up of Perform task range completion on shard rangeID update #4528 to make sure we won't call range completion with 0 (empty) exclusive max task key. Otherwise persistence layer will reject the request and cause queue processor to skip persisting its state.
• ^ issue only happens in very care case (task processing doesn't make any progress for a new clsuter) and even if it happens there's no actual impact. Once the problem on the task processing side is fixed, the errors will be gone automatically.

Why?

• see above

How did you test it?

• Added unit test.

Potential risks

Is hotfix candidate?