net: if: net_if_get_default: Return NULL if no interfaces configured

net_if_get_default() was documented as returning "Default interface
or NULL if no interfaces are configured.", but actually didn't
return NULL in the latter case. Instead, it effectively returned
a pointer to random area of memory, shared with other system
structures, so calling functions like net_if_ipv4_set_netmask(),
etc. could trash unrelated memory.

Jira: ZEP-2105

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>