Prevent generating EEOP_AGG_STRICT_INPUT_CHECK operations when nargs == 0.

Prevent generating EEOP_AGG_STRICT_INPUT_CHECK operations when nargs == 0.

This only became a problem with 4c640f4f38, which didn't synchronize
the value agg_strict_input_check.nargs is set to, with the guard
condition for emitting the operation.

Besides such instructions being unnecessary overhead, currently the
LLVM JIT provider doesn't support them. It seems more sensible to
avoid generating such instruction than supporting them. Add assertions
to make it easier to debug a potential further occurance.

Discussion: https://postgr.es/m/2a505161-2727-2473-7c46-591ed108ac52@email.cz
Backpatch: 11-, like 4c640f4f38.