repl: fix eval return value

In case no error has occurred during the evaluation of some code,
`undefined` has been returned in some cases as error argument instead
of `null`. This is fixed by this patch.

PR-URL: #25731
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Author: BridgeAR

lib/repl.js

@@ -216,10 +216,11 @@ function REPLServer(prompt,
   }
 
   function defaultEval(code, context, file, cb) {
-    var err, result, script, wrappedErr;
-    var wrappedCmd = false;
-    var awaitPromise = false;
-    var input = code;
+    let result, script, wrappedErr;
+    let err = null;
+    let wrappedCmd = false;
+    let awaitPromise = false;
+    const input = code;
 
     if (/^\s*\{/.test(code) && /\}\s*$/.test(code)) {
       // It's confusing for `{ a : 1 }` to be interpreted as a block
@@ -363,7 +364,7 @@ function REPLServer(prompt,
         }
 
         promise.then((result) => {
-          finishExecution(undefined, result);
+          finishExecution(null, result);
         }, (err) => {
           if (err && process.domain) {
             debug('not recoverable, send to domain');