Fix occasional Hypothesis test failure when calling get_or_create_collection

[levand]

Description of changes

Hypothesis tests would occasionally fail when they stumbled on a scenario like the following:

state = CollectionStateMachine(api)
state.initialize()
v1 = state.create_coll(coll={"name": "A11", "metadata": {"foo": "bar"}})
state.get_or_create_coll(coll={"name": "A11", "metadata": None})
state.teardown()

This PR allows the tests to pass by not making the assertion in the special case of metadata=None.

This cannot be fixed in code, because we have already designed passing metadata=None to mean "do not update the metadata." As such, there is no way to explicitly set the metadata to None without making user-facing API changes, and so the tests should not try to do so.

Test plan

This fixes a failure in the existing tests.

Documentation Changes

N/A

[HammadB]

Can we add a TODO to change this API behavior in the future.

[levand]

Can we add a TODO to change this API behavior in the future.

#380