The REFER I get:
   REFER sip:44555202@172.16.16.141:9781;transport=udp SIP/2.0
   Via: SIP/2.0/UDP 172.16.16.130:5060;branch=z9hG4bK-524287-1---cca7663c48d77f3a;rport
   Via: SIP/2.0/UDP 192.168.63.72:5061;branch=z9hG4bK-2b6d69c5
   Max-Forwards: 69
   Contact: <sip:44111200@192.168.63.72:5061>
   To: <sip:44555202@172.16.16.130>;tag=a1r2KaQ5Q3vUm
   From: <sip:44111200@172.16.16.130>;tag=33099d38e40eb484o1
   Call-ID: b0184248-ae01d2b4@192.168.63.72
   CSeq: 103 REFER
   User-Agent: Cisco/SPA504G-7.6.0
   Refer-To: <sip:44555204@172.16.16.130?Replaces=e1d8e9ed-f4fa54d8%40192.168.63.72%3Bfrom-tag%3D7fbc6ee9cfb6340co1%3Bto-tag%3DBajUN578mcKeg>
   Referred-By: <sip:44111200@172.16.16.130>
   Content-Length: 0

The part of RFC 3515 saying a body is not mandatory:
2.3 Message Body Inclusion

   A REFER method MAY contain a body.  This specification assigns no
   meaning to such a body.  A receiving agent may choose to process the
   body according to its Content-Type.



Original code not firing the event if the REFER does not have a body+content-type:
	if (sip->sip_content_type &&
		sip->sip_content_type->c_type && sip->sip_payload && sip->sip_payload->pl_data && !strcasecmp(sip->sip_event->o_type, "refer")) {
		if (switch_event_create_subclass(&s_event, SWITCH_EVENT_CUSTOM, MY_EVENT_NOTIFY_REFER) == SWITCH_STATUS_SUCCESS) {
			switch_event_add_header_string(s_event, SWITCH_STACK_BOTTOM, "content-type", sip->sip_content_type->c_type);
			switch_event_add_body(s_event, "%s", sip->sip_payload->pl_data);
		}
	}


What I think the code should be like to cope with a REFER without a body (not tested yet):
  if (!strcasecmp(sip->sip_event->o_type, "refer")) {
    if (switch_event_create_subclass(&s_event, SWITCH_EVENT_CUSTOM, MY_EVENT_NOTIFY_REFER) == SWITCH_STATUS_SUCCESS) {
      if (sip->sip_content_type && sip->sip_content_type->c_type && sip->sip_payload && sip->sip_payload->pl_data) {
        switch_event_add_header_string(s_event, SWITCH_STACK_BOTTOM, "content-type", sip->sip_content_type->c_type);
        switch_event_add_body(s_event, "%s", sip->sip_payload->pl_data);
      }
    }
  }