uma: allow uma_zfree_pcu(..., NULL)

We already allow free(NULL) and uma_zfree(..., NULL). Make
uma_zfree_pcpu(..., NULL) work as well.
This also means that counter_u64_free(NULL) will work.

These make cleanup code simpler.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29189