FI-3474 Add check if resource_type is nil

[yunwwang]

This fix GitHub Issue onc-healthit/onc-certification-g10-test-kit#589

Root cause for this issue is that resource_class crashed if reference and then resource_type is empty.

Solution:

Add a check if resource_type is nil then return nil

[Jammjammjamm]

Don't the other versions need to be updated as well?

[Jammjammjamm]

It should be possible to replace the multiple implementations of resource_class with a single implementation in ReferenceExtras which uses self.class.name.deconstantize.constantize instead of FHIR::WHATEVER_VERSION.const_get(resource_type).