fix: handle InMemoryDatabase find_schemas latest when no versions available for subject

[biggusdonzus]

This PR fixes an IndexError: list index out of range exception raised when calling InMemoryDatabase.find_schemas with argument latest_only = True and there's a schema with no versions.

[aiven-anton]

issue: As this test has a very specific purpose: verify we don't blow up when given an empty string, I think we should have more specific naming of this test. I find this to be the best way to structure test for maximizing communication of original intent:

class TestFindSchemas:  # <- _what_ are we testing
    def test_returns_empty_list_when_no_schemas(  # <- what _expectation_ do we have on the tested thing
        self,
    ) -> None:
        ... 
    # Future tests can easily be added here:
    def test_raises_foo_error_when_stars_are_not_aligned() -> None: ...

[aiven-anton]

Thanks, this is great. I could merge this as-is, but would suggest adding some more context to the test, such that future readers understand your intent.