Pass empty array instead of null to rbac list

The DataListToolbar component expects an array for additional controls
in all cases. When there's no controls to pass into the toolbar,
provide an empty array to avoid type-related errors.

awx/ui_next/src/components/ResourceAccessList/ResourceAccessList.jsx

@@ -203,7 +203,7 @@ class ResourceAccessList extends React.Component {
                         onClick={this.handleAddOpen}
                       />,
                     ]
-                  : null
+                  : []
               }
             />
           )}