stream: no need to initial er with false

initial `er` with false is unnecessarily.

PR-URL: #20607
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/_stream_writable.js

@@ -251,7 +251,7 @@ function writeAfterEnd(stream, cb) {
 // and undefined/non-string values are only allowed in object mode.
 function validChunk(stream, state, chunk, cb) {
   var valid = true;
-  var er = false;
+  var er;
 
   if (chunk === null) {
     er = new ERR_STREAM_NULL_VALUES();