net: iface: Allow NULL pointer to be handled in net_if_is_ip_offloaded

In case native interface is used with NET_OFFLOAD enabled, the
net_if_is_ip_offloaded() could lead to a crash, if called on unbound
net_context (i.e. with no iface assigned to net_context yet).

As since commit 40d22402 the net_context
allocated on offloaded interface is always assigned with the iface
pointer during its creation, it can be safely assumed that in case of
NULL pointer is provided we deal with a native interface. Therefore
instead of asserting on the iface pointer, indicate that the interface
is native when NULL pointer is provided.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
(cherry picked from commit 32bd3aa7)