net: coap: Remove resource pointer NULL check

This commit removes the resource pointer NULL check inside the resource
enumeration loop of the `coap_well_known_core_get` function because the
expression `(resource + 1)` will never evaluate to NULL (aka. 0).

This fixes the "comparison will always evaluate as ‘true’ for the
pointer operand" warning generated by the GCC 12.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>