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).

[arscan]

We ended up working around this issue in a different spot (specifically inferno-framework/us-core-test-kit#212); however, after discussion with the team we are hopeful we can get a fix in at this level so we'll leave this open for now.

[arscan]

Closing, will consider addressing in a separate ticket as part of broader improvement efforts. The immediate need for this was addressed in an inferno test kit specific fix as mentioned earlier.