Mojo javascript encoder: Empty string encoded as NULL.

Instead, encode null as NULL (and not just any falsy value) via
a == comparison with null (and use ==, not === to treat undefined
the same way as null).

BUG=407258

Review URL: https://codereview.chromium.org/511443002

Cr-Commit-Position: refs/heads/master@{#292221}

mojo/public/js/bindings/codec.js

@@ -316,7 +316,8 @@ define("mojo/public/js/bindings/codec", [
   };
 
   Encoder.prototype.encodeStructPointer = function(cls, val) {
-    if (!val) {
+    if (val == null) {
+      // Also handles undefined, since undefined == null.
       this.encodePointer(val);
       return;
     }
@@ -325,7 +326,8 @@ define("mojo/public/js/bindings/codec", [
   };
 
   Encoder.prototype.encodeArrayPointer = function(cls, val) {
-    if (!val) {
+    if (val == null) {
+      // Also handles undefined, since undefined == null.
       this.encodePointer(val);
       return;
     }
@@ -337,7 +339,8 @@ define("mojo/public/js/bindings/codec", [
   };
 
   Encoder.prototype.encodeStringPointer = function(val) {
-    if (!val) {
+    if (val == null) {
+      // Also handles undefined, since undefined == null.
       this.encodePointer(val);
       return;
     }