app: define values for default_dispvm=None, updatevm=None
app: define values for default_dispvm=None, updatevm=None

If those VMs are not set, default to None, instead of throwing
AttributeError.

Fixes QubesOS/qubes-issues#3475

marmarek/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin