Handle case where we don't have a nick for an absent occupant

updates #1099

src/converse-muc.js

@@ -995,6 +995,10 @@
                     if (hash && vcard.get('image_hash') !== hash) {
                         _converse.api.vcard.update(vcard);
                     }
+                },
+
+                getDisplayName () {
+                    return this.get('nick') || this.get('jid');
                 }
             });
 
@@ -1006,8 +1010,8 @@
                     const role1 = occupant1.get('role') || 'none';
                     const role2 = occupant2.get('role') || 'none';
                     if (MUC_ROLE_WEIGHTS[role1] === MUC_ROLE_WEIGHTS[role2]) {
-                        const nick1 = occupant1.get('nick').toLowerCase();
-                        const nick2 = occupant2.get('nick').toLowerCase();
+                        const nick1 = occupant1.getDisplayName().toLowerCase();
+                        const nick2 = occupant2.getDisplayName().toLowerCase();
                         return nick1 < nick2 ? -1 : (nick1 > nick2? 1 : 0);
                     } else  {
                         return MUC_ROLE_WEIGHTS[role1] < MUC_ROLE_WEIGHTS[role2] ? -1 : 1;