module: use undefined if no main

If the package.json file does not have a "main" entry, return undefined
rather than an empty string. This is to make more consistent behavior.
For example, when package.json is a directory, "main" is undefined
rather than an empty string.

PR-URL: nodejs#18593
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Benedikt Meurer <benedikt.meurer@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

src/node_file.cc

@@ -652,9 +652,8 @@ void Close(const FunctionCallbackInfo<Value>& args) {
 
 
 // Used to speed up module loading.  Returns the contents of the file as
-// a string or undefined when the file cannot be opened.  Returns an empty
-// string when the file does not contain the substring '"main"' because that
-// is the property we care about.
+// a string or undefined when the file cannot be opened or "main" is not found
+// in the file.
 static void InternalModuleReadJSON(const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
   uv_loop_t* loop = env->event_loop();
@@ -708,7 +707,7 @@ static void InternalModuleReadJSON(const FunctionCallbackInfo<Value>& args) {
 
   const size_t size = offset - start;
   if (size == 0 || size == SearchString(&chars[start], size, "\"main\"")) {
-    args.GetReturnValue().SetEmptyString();
+    return;
   } else {
     Local<String> chars_string =
         String::NewFromUtf8(env->isolate(),

test/parallel/test-module-binding.js

@@ -6,8 +6,9 @@ const { readFileSync } = require('fs');
 const { strictEqual } = require('assert');
 
 strictEqual(internalModuleReadJSON('nosuchfile'), undefined);
-strictEqual(internalModuleReadJSON(fixtures.path('empty.txt')), '');
-strictEqual(internalModuleReadJSON(fixtures.path('empty-with-bom.txt')), '');
+strictEqual(internalModuleReadJSON(fixtures.path('empty.txt')), undefined);
+strictEqual(internalModuleReadJSON(fixtures.path('empty-with-bom.txt')),
+            undefined);
 {
   const filename = fixtures.path('require-bin/package.json');
   strictEqual(internalModuleReadJSON(filename), readFileSync(filename, 'utf8'));