codegen should issue a diagnostic for a present but blank `name` on a field

[ab9rf]

alternatively, it should delete them and let the usual anon handling take place

right now, if an field element has name='' then codegen.out.xml will have a blank name attribute but no ld:anon-name attribute (as is generated if name is absent). while the headers get an anon name in the expected manner, codegen.out.xml fails to reflect what is generated in headers, which shouldn't happen

related discord discussion

[lethosor]

We should probably also disallow this in the schema, if that's possible.