Add ClassFinder::namespaceHasClasses instead of throwing on empty namespaces

Changes

This PR changes the behaviour of ClassFinder::getClassesForNamespace to simply return an empty list of classes if there aren't any present in a given namespace.

I added ClassFinder::namespaceHasClasses to determine if a namespace contains any classes at all.

Reasoning

This has come up in a project of ours: https://github.com/nuwave/lighthouse/pull/948

Since we are scanning a configured directory for user-defined classes, we can not know in advance if there are any.