fix: returning a plain WP_User if guest authors is not enabled.

I forgot to run tests on my previous commit. This satisfies the test Test_CoAuthors_Plus::test_get_coauthor_by_when_guest_authors_not_enabled which is expecting a WP_User when the plugin is not enabled.

co-authors-plus.php

@@ -334,9 +334,17 @@ public function get_coauthor_by( $key, $value, $force = false ) {
 
 				return $user;
 			}
-		}
+		} else {
+			$user = $this->get_user_by( $key, $value );
 
-		return false;
+			if ( is_null( $user ) ) {
+				return false;
+			}
+
+			$user->type = 'wpuser';
+
+			return $user;
+		}
 	}
 
 	/**