[webnfc] Make the retval of NDEFRecord#{toText,toArrayBuffer,toJSON}() nullable

Previously, for invalid cases, toText() returns an empty string,
toJSON() returns undefined, which does not match the spec.

The spec was changed at:
w3c/web-nfc#298

BUG=520391

Change-Id: Ia63e675c5b0032cee13802d1e5d706c35089a056
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761586
Reviewed-by: Rijubrata Bhaumik <[email protected]>
Reviewed-by: Reilly Grant <[email protected]>
Commit-Queue: Leon Han <[email protected]>
Cr-Commit-Position: refs/heads/master@{#688840}

Author: Leon Han

web-nfc/NDEFMessage_constructor.https.html

@@ -28,8 +28,8 @@
         'toText() contains the same text content');
     assert_equals(message.records[0].toArrayBuffer(), null,
         'toArrayBuffer() returns null');
-    assert_equals(message.records[0].toJSON(), undefined,
-        'toJSON() returns undefined');
+    assert_equals(message.records[0].toJSON(), null,
+        'toJSON() returns null');
   }, 'NDEFMessage constructor with a text record');
 
 </script>

web-nfc/NDEFRecord_constructor.https.html

@@ -15,9 +15,9 @@
     const record = new NDEFRecord(null);
     assert_equals(record.recordType.length, 0, 'empty recordType');
     assert_equals(record.mediaType.length, 0, 'empty mediaType');
-    assert_equals(record.toText().length, 0, 'toText() gets an empty string');
+    assert_equals(record.toText(), null, 'toText() returns null');
     assert_equals(record.toArrayBuffer(), null, 'toArrayBuffer() returns null');
-    assert_equals(record.toJSON(), undefined, 'toJSON() returns undefined');
+    assert_equals(record.toJSON(), null, 'toJSON() returns null');
   }, 'NDEFRecord constructor with null init dict');
 
   test(() => {

web-nfc/resources/nfc-helpers.js

@@ -165,8 +165,8 @@ function assertWebNDEFMessagesEqual(message, expectedMessage) {
       expectedJson = expectedRecord.toJSON();
     } catch (e) {
     }
-    if (json === undefined)
-      assert_equals(expectedJson, undefined);
+    if (json === undefined || json === null)
+      assert_equals(json, expectedJson);
     else
       assert_object_equals(json, expectedJson);
   }