Prevent logging actions for instances with null primary keys
Prevent logging actions for instances with null primary keys

It's a common gotcha when logging delete actions, and screws up reporting afterwards because the generic FK in ModelLogEntry ends up storing it as the string 'None'

wagtail/wagtail,mixxorz/wagtail,rsalmaso/wagtail,zerolab/wagtail,rsalmaso/wagtail,wagtail/wagtail,thenewguy/wagtail,zerolab/wagtail,thenewguy/wagtail,mixxorz/wagtail,wagtail/wagtail,mixxorz/wagtail,rsalmaso/wagtail,torchbox/wagtail,jnns/wagtail,torchbox/wagtail,zerolab/wagtail,jnns/wagtail,torchbox/wagtail,torchbox/wagtail,rsalmaso/wagtail,thenewguy/wagtail,mixxorz/wagtail,wagtail/wagtail,zerolab/wagtail,thenewguy/wagtail,wagtail/wagtail,rsalmaso/wagtail,zerolab/wagtail,thenewguy/wagtail,jnns/wagtail,mixxorz/wagtail,jnns/wagtail