Bugfix: Handle empty messages correctly.

Author: muukong

src/main/java/com/muukong/protobuf/PBMessage.java

@@ -57,7 +57,7 @@ public String prettyPrint(String indent) {
     public byte[] serializeValue() {
 
         if ( fields.size() == 0 ) { // Empty message
-            return new byte[] { 0, 0, 0, 0, 0 };
+            return new byte[0];
         }
 
         List<Byte> result = new ArrayList<>();

src/test/java/com/muukong/protobuf/PBDisassemblerTest.java

@@ -85,6 +85,23 @@ void emptyInput_producesEmptyMessage() {
         assertEquals("", pp(new byte[0]));
     }
 
+    @Test
+    void emptyMessage_serializesToZeroBytes() {
+        PBMessage empty = new PBDisassembler(new byte[0]).disassemble();
+        assertArrayEquals(new byte[0], empty.serializeValue());
+    }
+
+    @Test
+    void emptySubMessage_roundTrip() {
+        // A LEN field whose payload is an empty protobuf message (0 bytes).
+        // Before the fix, PBMessage.serializeValue() returned 5 zero bytes for
+        // empty messages, so this round-trip would produce a 5-byte payload instead.
+        byte[] input = lenField(1, new byte[0]);   // field 1, length 0 → empty string {""}
+        // The disassembler treats a zero-length LEN as an empty string.
+        // Serialising that PBString back must reproduce the original 2-byte encoding.
+        assertArrayEquals(input, new PBDisassembler(input).disassemble().serializeValue());
+    }
+
     // -----------------------------------------------------------------------
     // VARINT fields
     // -----------------------------------------------------------------------