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.

tools/test-js-with-node

@@ -231,7 +231,6 @@ EXEMPT_FILES = make_set(
         "web/src/zulip.js",
         "web/src/zulip_test.js",
         "web/tests/lib/mdiff.js",
-        "web/tests/lib/zjquery_element.js",
     ]
 )
 

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();