samples/coaps_server: Don't error if the packet doesn't have payload

GET and DEL requests may not have payloads, so it's not correct to
return with an error in those cases.

This was only noticed now that zoap_packet_get_payload() returns the
correct value for all situations.

Jira: ZEP-1754

Change-Id: Ie533041aa7a66855582ff4c5c937d943304bad84
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>