MetadataGenerationService: clean up reginfo code

Clean up the Registration and Publishing Information extension to the
MetadataGenerationService as per comments by @braddleybeddoes in #157 -
specifically:
* use "x && !x.isEmpty()" to test if a string is set to a non-empty value
* correctly reference local variables instead of the fields in config
* drop unnecessary explicit "return" in front of the return value
  in the populateSchema function
MetadataGenerationService: refer to RPI XSD

Add the reference to the Registration and Publishing Info XSD in the
generated metadata (if the RPI namespace is actually used).

The XSD file itself can be downloaded from
http://docs.oasis-open.org/security/saml/Post2.0/saml-metadata-rpi/v1.0/cs01/xsd/saml-metadata-rpi-v1.0.xsd
- and it is the responsibility of interested parties to deploy the XSD
locally to validate the mdrpi extensions.

Adjust the test-case data accordingly.