lib: make validateAbortSignal stricter

We should not force-check if the passed signal is **NOT** undefined
and then proceed to check whether the value is an abort signal or not
as the validator won't throw an error if the value is undefined as we
straight up validate the values instead of checking if they're not
undefined first. This unnecessary check can be done explicitly outside
of the validator to only pass the value to the validator if its not
undefined.

lib/child_process.js

@@ -690,7 +690,9 @@ function abortChildProcess(child, killSignal) {
 function spawn(file, args, options) {
   options = normalizeSpawnArguments(file, args, options);
   validateTimeout(options.timeout);
-  validateAbortSignal(options.signal, 'options.signal');
+  if (options.signal !== undefined)
+    validateAbortSignal(options.signal, 'options.signal');
+
   const killSignal = sanitizeKillSignal(options.killSignal);
   const child = new ChildProcess();
 

lib/internal/fs/promises.js

@@ -813,7 +813,8 @@ async function writeFile(path, data, options) {
     data = Buffer.from(data, options.encoding || 'utf8');
   }
 
-  validateAbortSignal(options.signal);
+  if (options.signal !== undefined)
+    validateAbortSignal(options.signal);
   if (path instanceof FileHandle)
     return writeFileHandle(path, data, options.signal, options.encoding);
 

lib/internal/validators.js

@@ -224,10 +224,8 @@ const validateCallback = hideStackFrames((callback) => {
 });
 
 const validateAbortSignal = hideStackFrames((signal, name) => {
-  if (signal !== undefined &&
-      (signal === null ||
-       typeof signal !== 'object' ||
-       !('aborted' in signal))) {
+  if (signal === null || typeof signal !== 'object' ||
+      !('aborted' in signal)) {
     throw new ERR_INVALID_ARG_TYPE(name, 'AbortSignal', signal);
   }
 });