Fix message stanza with body changing MUC subject (dino#1569)

- fixes dino#1542
            - more consistent with
              https://xmpp.org/extensions/xep-0045.html#enter-subject:
                    "Note: In accordance with the core definition of XML stanzas,
                    any message can contain a <subject/> element; only a message that
                    contains a <subject/> but no <body/> element shall be considered a
                    subject change for MUC purposes."

xmpp-vala/src/module/xep/0045_muc/module.vala

@@ -285,7 +285,7 @@ public class Module : XmppStreamModule {
     private void on_received_message(XmppStream stream, MessageStanza message) {
         if (message.type_ == MessageStanza.TYPE_GROUPCHAT) {
             StanzaNode? subject_node = message.stanza.get_subnode("subject");
-            if (subject_node != null) {
+            if (subject_node != null && message.body == null){ 
                 string subject = subject_node.get_string_content();
                 stream.get_flag(Flag.IDENTITY).set_muc_subject(message.from, subject);
                 subject_set(stream, subject, message.from);