replication: add IsEmpty method to GTIDSet interface & implementations

[wesleywright]

Small quality of life improvement. For some use cases (such as writing tests), it is common to want to check if a given GTID set is empty or not. Currently this requires using Equal() with an empty GTID set (which takes a decent amount of boilerplate) or else requires inspecting the Set field (which is specific to a single concrete implementation of the GTIDSet interface).

This commits adds a simple IsEmpty() bool method to the GTIDSet interface for this use case, as well as implementations for both MysqlGTIDSet and MariadbGTIDSet.

[wesleywright]

This is my first contribution to this repo; it seemed like a pretty small and straightforward change so I thought it was simplest to directly submit it as a PR. I'm happy to take any feedback on it :-)