Skip rdfParse if script content is empty

[csarven]

May be sufficient to resolve https://github.com/linkeddata/rdflib.js/issues/692 . There should be better checks before committing to parse, e.g., type=text/html and application/xhtml+xml in a script block is not particularly common.

[jeswr]

@csarven does this still need merging? @doriantaylor do you have a test case you can add?

[doriantaylor]

I didn't originally but I could I suppose; is there a guideline for adding tests somewhere?