Failure when AnyFixture reporting is enabled but AnyFixture has not been used.

[Systho]

https://github.com/palkan/test-prof/blob/e135f2df9fff257d7c3944707f46bb960373528c/lib/test_prof/any_fixture.rb#L90

When reporting is enabled, AnyFixture will introspect its cache after suite has run.
But if AnyFixture has not been used, the cache will be empty and this will return nil :

cache.stats.keys.map(&:size).max

Then nil + 2 will throw an error.

I think the report_stats method should return immediately if cache is empty with a simple message "No AnyFixture Stats available".