metadata: fix segfault when filters reject devices

metadata: fix segfault when filters reject devices

Checking for devices uses is_missing_pv() to check
if there is a device for the PV.  is_missing_pv()
is based on the MISSING_PV flag, which does not
always correspond to !pv->dev.  When using lvmetad,
a command like:

pvs --config 'devices/filter=["a|/dev/sdb|", "r|.*|"]'

will cause a number of PVs to have NULL pv->dev, but
not the MISSING_PV flag.  So, NULL pv->dev needs to
also be checked.