Ensure that role is not empty before adding it in add_role() function and methods.

Props MikeHansenMe, dannydehaan, michielhab.
Fixes #23746.


git-svn-id: https://develop.svn.wordpress.org/trunk@33967 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-user.php

@@ -439,6 +439,10 @@ class WP_User {
 	 * @param string $role Role name.
 	 */
 	public function add_role( $role ) {
+		if ( empty( $role ) ) {
+			return;
+		}
+
 		$this->caps[$role] = true;
 		update_user_meta( $this->ID, $this->cap_key, $this->caps );
 		$this->get_role_caps();