net: lwm2m: remove IP CONFIG checks in lwm2m_sprint_ip_addr()

CONFIG_NET_IPV* checks are not needed in lwm2m_sprint_ip_addr().  The
functions used are always available.  Worse, having these checks
forces the need to enable CONFIG_NET_IPV4 or IPV6 when it's not really
needed (LwM2M could be using an offloaded IP stack).

NOTE: Also fixes an issue where a NULL is returned when the IP address
is unknown.  This usually ends up with a crash/abort in the logging
code.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/17401



Signed-off-by: Michael Scott <mike@foundries.io>