fix: Scan on non-existing key should return zero cursor

[mkaruza]

Fixes #5777

[Niennienzz]

Thanks for the fix! @mkaruza

Without understanding the code, I would like to double-check and make sure that this behavior of scanning is preserved.

Basically, during a full iteration:

The scan commands are also allowed to return zero elements, and the client should not consider the iteration complete as long as the returned cursor is not zero.

^ In that case, the client should continue scanning.

Unlike the non-existing key case described in the issue, where the client should stop as it gets cursor 0.

[mkaruza]

@Niennienzz can you let me know after you double check if this is correct