[mlir:PDLL] Don't require users to provide operands/results when all are variadic

When all operands or results are variadic, zero values is a perfectly valid behavior
to expect, and we shouldn't force the user to provide values in this case. For example,
when creating a call or a return operation we often don't want/need to provide return
values.

Differential Revision: https://reviews.llvm.org/D133721