net: coap: Fix coap_packet_is_request() check for empty code

Empty code was incorrectly matched as a request, fix that.

Align coap_handle_request_len() function to behave as documented in the
API documentation - in case of invalid request code (which is also the
case for empty code) -ENOTSUP Should be returned.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>