stream: simplify Readable push/unshift logic

PR-URL: #32899
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/_stream_readable.js

@@ -230,13 +230,15 @@ function readableAddChunk(stream, chunk, encoding, addToFront) {
   if (!state.objectMode) {
     if (typeof chunk === 'string') {
       encoding = encoding || state.defaultEncoding;
-      if (addToFront && state.encoding && state.encoding !== encoding) {
-        // When unshifting, if state.encoding is set, we have to save
-        // the string in the BufferList with the state encoding.
-        chunk = Buffer.from(chunk, encoding).toString(state.encoding);
-      } else if (encoding !== state.encoding) {
-        chunk = Buffer.from(chunk, encoding);
-        encoding = '';
+      if (state.encoding !== encoding) {
+        if (addToFront && state.encoding) {
+          // When unshifting, if state.encoding is set, we have to save
+          // the string in the BufferList with the state encoding.
+          chunk = Buffer.from(chunk, encoding).toString(state.encoding);
+        } else {
+          chunk = Buffer.from(chunk, encoding);
+          encoding = '';
+        }
       }
     } else if (chunk instanceof Buffer) {
       encoding = '';