Many elements in SAML metadata are given the basic string type in the schema but are further constrained by section 1.3.1 of SAMLCore as follows:

Unless otherwise noted in this specification or particular profiles, all strings in SAML messages MUST consist of at least one non-whitespace character (whitespace is defined in the XML Recommendation [XML] Section 2.3).

This can be done pretty easily using the DOM traversal framework I've been developing in the ukf-mda project, so I will prototype this there. The obvious API is to allow a Collection<QName> to check.