child_process: remove empty if condition

This commit replaces an empty if-else-if with a single if
condition.

PR-URL: https://github.com/nodejs/node/pull/11427
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

lib/child_process.js

@@ -187,9 +187,7 @@ exports.execFile = function(file /*, args, options, callback*/) {
       stderr = Buffer.concat(_stderr);
     }
 
-    if (ex) {
+    if (!ex && code === 0 && signal === null) {
-      // Will be handled later
-    } else if (code === 0 && signal === null) {
       callback(null, stdout, stderr);
       return;
     }