Set default value of 'status' to `undefined`.

When no status message is in a presence stanza, the value gets set to
undefined, so we might as well make this the default value to avoid
unnecessarily triggering change events.

dist/converse.js

@@ -68784,7 +68784,7 @@ _converse_headless_converse_core__WEBPACK_IMPORTED_MODULE_0__["default"].plugins
         'image': _converse.DEFAULT_IMAGE,
         'image_type': _converse.DEFAULT_IMAGE_TYPE,
         'num_unread': 0,
-        'status': ''
+        'status': undefined
       },
 
       initialize(attributes) {

src/headless/converse-roster.js

@@ -229,7 +229,7 @@ converse.plugins.add('converse-roster', {
                 'image': _converse.DEFAULT_IMAGE,
                 'image_type': _converse.DEFAULT_IMAGE_TYPE,
                 'num_unread': 0,
-                'status': '',
+                'status': undefined,
             },
 
             initialize (attributes) {