vm_phys_enq_range: no alignment assert for npages==0

Do not assume that when vm_phys_enq_range is passed npages==0 that the
vm_page argument is valid in any way, much less that it has a
page-aligned address. Just don't look at it. Assert nothing about it.

Reported by: karels
Differential Revision: https://reviews.freebsd.org/D41317