Retain mculture when clicking results from global search (#7192)

Author: Kenn Jacobsen

src/Umbraco.Web.UI.Client/src/common/services/navigation.service.js

@@ -259,8 +259,8 @@ function navigationService($routeParams, $location, $q, $injector, eventsService
             var updated = false;
 
             retainedQueryStrings.forEach(r => {
-                // if mculture is set to null in nextRouteParams, the value will be undefined and we will not retain any query string that has a value of "null"
-                if (currRouteParams[r] && nextRouteParams[r] !== undefined && !nextRouteParams[r]) {
+                // testing explicitly for undefined in nextRouteParams here, as it must be possible to "unset" e.g. mculture by specifying a null value
+                if (currRouteParams[r] && nextRouteParams[r] === undefined) {
                     toRetain[r] = currRouteParams[r];
                     updated = true;
                 }