net: Build assert issue with llvm

Remove the build assert from NET_L3_REGISTER() macro as that
is causing an issue with llvm. Add runtime check of the handler
pointer value.

subsys/net/l2/ethernet/arp.c:1044:1: error: static_assert expression
is not an integral constant expression

ETH_NET_L3_REGISTER(ARP, NET_ETH_PTYPE_ARP, arp_recv);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/zephyr/net/ethernet.h:1272:2: note: expanded from
macro 'ETH_NET_L3_REGISTER'
        NET_L3_REGISTER(&NET_L2_GET_NAME(ETHERNET), name, ptype, handler)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/zephyr/net/net_core.h:190:2: note: expanded from
macro 'NET_L3_REGISTER'
        BUILD_ASSERT((_handler) != NULL, "Handler is not defined")
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
include/zephyr/toolchain/gcc.h:87:51: note: expanded from
macro 'BUILD_ASSERT'
define BUILD_ASSERT(EXPR, MSG...) _Static_assert((EXPR), "" MSG)
                                                  ^~~~~~
subsys/net/l2/ethernet/arp.c:1044:1: note: cast from 'void *' is not
allowed in a constant expression
include/zephyr/net/ethernet.h:1272:2: note: expanded from
macro 'ETH_NET_L3_REGISTER'
        NET_L3_REGISTER(&NET_L2_GET_NAME(ETHERNET), name, ptype, handler)
        ^
include/zephyr/net/net_core.h:190:29: note: expanded from
macro 'NET_L3_REGISTER'
        BUILD_ASSERT((_handler) != NULL, "Handler is not defined")
                                   ^
/usr/lib/llvm-14/lib/clang/14.0.0/include/stddef.h:89:16: note: expanded
from macro 'NULL'
  define NULL ((void*)0)

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>