Fix: 1802 [Bug] Math Equation would be null. #1802 (#1806)

* fix: #1290 [Bug] 300ms delay on buttons in titlebar

* fix: #1802 Math Equation would be null

* fix: retain  as a attribute value

---------

Co-authored-by: nathan <nathan@appflowy.io>

frontend/rust-lib/flowy-document/tests/editor/attribute_test.rs

@@ -775,7 +775,7 @@ fn delta_compose() {
     }
     assert_eq!(
         delta.json_str(),
-        r#"[{"insert":"a"},{"insert":"\n","attributes":{"list":"unchecked"}},{"insert":"\n"}]"#
+        r#"[{"insert":"a"},{"insert":"\n","attributes":{"list":"unchecked"}},{"insert":"\n","attributes":{"list":""}}]"#
     );
 
     let ops = vec![