Revert "Issue #3416141 by gorkagr, Jaypan, Wim Leers: big_pipe.js'...

Revert "Issue #3416141 by gorkagr, Jaypan, Wim Leers: big_pipe.js' checkMutation() does not check if node exists before using it"

This reverts commit 2d17900f.

core/modules/big_pipe/js/big_pipe.js

@@ -89,8 +89,7 @@
    */
   function checkMutation(node) {
     return Boolean(
-      node &&
-        node.nodeType === Node.ELEMENT_NODE &&
+      node.nodeType === Node.ELEMENT_NODE &&
         node.nodeName === 'SCRIPT' &&
         node.dataset &&
         node.dataset.bigPipeReplacementForPlaceholderWithId &&