don't die when the assigned_to field is not set on a WP

app/assets/javascripts/angular/work_packages/directives/work-package-column-directive.js

@@ -125,7 +125,7 @@ angular.module('openproject.workPackages.directives')
           case 'work_package':
             return PathHelper.workPackagePath(id);
           case 'user':
-            if (scope.workPackage[scope.column.name].type == 'Group') {
+            if (scope.workPackage[scope.column.name] && scope.workPackage[scope.column.name].type == 'Group') {
               // if it's a group, we have nothing to link to
               scope.displayType = 'text';
               return '';