compose: Remove empty message warning border after closing.

Adding this made the line coverage of
"web/tests/lib/zjquery_element.js" equal to 100%, hence it
was required to remove it from the exempt files list.

web/src/compose_actions.js

@@ -118,6 +118,7 @@ function clear_box() {
     clear_textarea();
     compose_validate.check_overflow_text();
     $("#compose-textarea").removeData("draft-id");
+    $("#compose-textarea").toggleClass("invalid", false);
     compose_ui.autosize_textarea($("#compose-textarea"));
     compose_banner.clear_errors();
     compose_banner.clear_warnings();