How to query SOLR for empty fields?
Try this:
?q=-id:["" TO *]
One caveat! If you want to compose this via OR or AND you cannot use it in this form:
-myfield:*
but you must use
(*:* NOT myfield:*)
This form is perfectly composable. Apparently SOLR will expand the first form to the second, but only when it is a top node. Hope this saves you some time!