config: pending network: avoid undef-warning on old/new comparison

config: pending network: avoid undef-warning on old/new comparison

A network device of a VM does not necessarily has to be connected to
an actual bridge, so when a new pending value is set we need to use
the undef-safe compare helpers when checking if there was a change
between old and new value, as otherwise one gets ugly "use of
uninitialized value in string ne" warnings.

Link: https://forum.proxmox.com/threads/143072/
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>