use null instead of false in data that could be sent to api

assets/less/bootstrap/mixins/tab-focus.less

@@ -1,9 +1,9 @@
 // WebKit-style focus
 
 .tab-focus() {
-  // WebKit-specific. Other browsers will keep their default outline style.
-  // (Initially tried to also force default via `outline: initial`,
-  // but that seems to erroneously remove the outline in Firefox altogether.)
+  // Default
+  outline: thin dotted;
+  // WebKit
   outline: 5px auto -webkit-focus-ring-color;
   outline-offset: -2px;
 }