Checkpoint might be empty

[Tristan1900]

Describe the bug
A Stream names that the reader group can read from cannot be empty error happened during Flink longevity run. After checking the code, it might be the case that initiateCheckpoint returns a checkpoint that does not contain any stream information. Currently there is no guarantee that the checkpoint cannot be empty, it's better to add an assert clause to fail fast before it returns to the caller.

To Reproduce
Flink longevity run. It looks like a rare case, no clear steps to reproduce.