Don't validate type for blank values

Another case where a validation is implying ValuePresence when it shouldn't be.
This should be the last case as now all the nodes that use ValuePresence have
been fixed.
What is the intention here of returning `nil`? Is it to trigger https://github.com/fac/hermod/blob/57d3d9a8b1dd524fa6d064f6c99217794aa33ebb/lib/hermod/xml_section_builder.rb#L202 ?

It doesn't feel very clear. You're still passing the attributes through, but you're not expecting any mutation to be processed on the values?