cubicweb #3670211 RRQLExpression.check returns False when the context is incomplete [open]

When S (resp. O) are in defined variables of the RRQLExpression, check method will return False if the fromeid (resp. toeid) are not specified. So, first, I think this behavior should be documented, or at least commented in the code. Then, it may generate false negative results when the caller does not supply enough context, see https://www.cubicweb.org/ticket/3670209 for a possible consequence of this behavior.

Maybe a warning should be emitted.