Avoid adding empty splices in CodedBufferWriter (#886)

By avoiding allocating empty `Uint8List`s this saves a few percent in dart2js
-O4 in the benchmark reported in #885.

|                              | Before     | After      | Diff                |
|------------------------------|------------|------------|---------------------|
| AOT                          | 122,917 us | 122,741 us |                     |
| JIT                          |  93,376 us |  94,880 us |                     |
| dart2js -O4                  | 271,111 us | 258,250 us | -12,861 us, -4.7%   |
| dart2wasm --omit-type-checks | 196,454 us | 195,300 us |                     |

Number of splices in the benchmark before this change: 21,123
After: 20,857

protobuf/lib/src/protobuf/coded_buffer_writer.dart

@@ -226,9 +226,11 @@ class CodedBufferWriter {
   /// Add a [Uint8List] splice, without copying. These bytes will be directly
   /// copied into the output buffer by [writeTo].
   void writeRawBytes(Uint8List value) {
+    final length = value.lengthInBytes;
+    if (length == 0) return;
     _commitSplice();
     _splices.add(value);
-    _bytesTotal += value.lengthInBytes;
+    _bytesTotal += length;
   }
 
   /// Start writing a length-delimited data.
@@ -340,11 +342,22 @@ class CodedBufferWriter {
         _writeVarint32(value ? 1 : 0);
         break;
       case PbFieldType._BYTES_BIT:
-        _writeBytesNoTag(
-            value is Uint8List ? value : Uint8List.fromList(value));
+        final List<int> bytes = value;
+        if (bytes is Uint8List) {
+          _writeBytesNoTag(bytes);
+        } else if (bytes.isEmpty) {
+          _writeEmptyBytes();
+        } else {
+          _writeBytesNoTag(Uint8List.fromList(bytes));
+        }
         break;
       case PbFieldType._STRING_BIT:
-        _writeBytesNoTag(const Utf8Encoder().convert(value));
+        final String string = value;
+        if (string.isEmpty) {
+          _writeEmptyBytes();
+        } else {
+          _writeBytesNoTag(const Utf8Encoder().convert(string));
+        }
         break;
       case PbFieldType._DOUBLE_BIT:
         _writeDouble(value);
@@ -405,6 +418,10 @@ class CodedBufferWriter {
     writeRawBytes(value);
   }
 
+  void _writeEmptyBytes() {
+    writeInt32NoTag(0);
+  }
+
   void _writeTag(int fieldNumber, int wireFormat) {
     writeInt32NoTag(makeTag(fieldNumber, wireFormat));
   }