Rename empty bytes in JavaNano.

javanano/src/main/java/com/google/protobuf/nano/InternalNano.java

@@ -352,13 +352,13 @@ public final class InternalNano {
     }
     return result;
   }
-  private static final byte[] emptyBytes = new byte[0];
+  private static final byte[] EMPTY_BYTES = new byte[0];
   private static Object primitiveDefaultValue(int type) {
     switch (type) {
       case TYPE_BOOL:
         return Boolean.FALSE;
       case TYPE_BYTES:
-        return emptyBytes;
+        return EMPTY_BYTES;
       case TYPE_STRING:
         return "";
       case TYPE_FLOAT:
@@ -443,7 +443,7 @@ public final class InternalNano {
     }
 
     if (value == null) {
-      // message type
+      // message type value will be initialized by code-gen.
       value = (V) primitiveDefaultValue(valueType);
     }
 

javanano/src/test/java/com/google/protobuf/nano/NanoTest.java

@@ -3742,7 +3742,6 @@ public class NanoTest extends TestCase {
     byte[] output = MessageNano.toByteArray(origin);
     TestMap parsed = new TestMap();
     MessageNano.mergeFrom(parsed, output);
-    // TODO(liujisi): Test missing key/value in parsing.
   }
 
   public void testMapSerializeRejectNull() throws Exception {