net: coap: empty payload with payload marker is malformed

From RFC 7252, section 3
"The absence of the Payload Marker denotes a zero-length payload.
The presence of a marker followed by a zero-length payload MUST
be processed as a message format error."

Check empty payload when COAP_MARKER is found and add a test case to
cover it

Signed-off-by: Robert Chou <robert.ch.chou@acer.com>