Add connected check after firing command event

Tests: web-platform-tests/wpt#50871.

Fixes #11043.

Author: lukewarlow

source

@@ -53666,6 +53666,8 @@ interface <dfn interface>HTMLButtonElement</dfn> : <span>HTMLElement</span> {
 
      <li><p>If <var>continue</var> is false, then return.</p></li>
 
+     <li><p>If <var>target</var> is not <span>connected</span>, then return.</p></li>
+
      <li><p>If <var>command</var> is in the <span
      data-x="attr-button-command-custom-state">Custom</span> state, then return.</p></li>