Move Count checks into the serialization code

I wouldn't expect this to affect anything, but it appears to.

src/google/protobuf/compiler/csharp/csharp_repeated_enum_field.cc

@@ -80,11 +80,11 @@ void RepeatedEnumFieldGenerator::GenerateParsingCode(io::Printer* printer) {
 }
 
 void RepeatedEnumFieldGenerator::GenerateSerializationCode(io::Printer* printer) {
-  // TODO(jonskeet): Originally, this checked for Count > 0 first.
-  // The Write* call should make that cheap though - no need to generate it every time.
   printer->Print(
     variables_,
-    "output.Write$packed$EnumArray($number$, $name$_);\n");
+    "if ($name$_.Count > 0) {\n"
+    "  output.Write$packed$EnumArray($number$, $name$_);\n"
+    "}\n");
 }
 
 void RepeatedEnumFieldGenerator::GenerateSerializedSizeCode(io::Printer* printer) {

src/google/protobuf/compiler/csharp/csharp_repeated_message_field.cc

@@ -83,7 +83,9 @@ void RepeatedMessageFieldGenerator::GenerateSerializationCode(io::Printer* print
   // The Write* call should make that cheap though - no need to generate it every time.
   printer->Print(
     variables_,
-    "output.WriteMessageArray($number$, $name$_);\n");
+    "if ($name$_.Count > 0) {\n"
+    "  output.WriteMessageArray($number$, $name$_);\n"
+    "}\n");
 }
 
 void RepeatedMessageFieldGenerator::GenerateSerializedSizeCode(io::Printer* printer) {

src/google/protobuf/compiler/csharp/csharp_repeated_primitive_field.cc

@@ -49,6 +49,7 @@ namespace csharp {
 RepeatedPrimitiveFieldGenerator::RepeatedPrimitiveFieldGenerator(
     const FieldDescriptor* descriptor, int fieldOrdinal)
     : FieldGeneratorBase(descriptor, fieldOrdinal) {
+  variables_["packed"] = descriptor->is_packed() ? "Packed" : "";
 }
 
 RepeatedPrimitiveFieldGenerator::~RepeatedPrimitiveFieldGenerator() {
@@ -79,15 +80,11 @@ void RepeatedPrimitiveFieldGenerator::GenerateParsingCode(io::Printer* printer)
 
 void RepeatedPrimitiveFieldGenerator::GenerateSerializationCode(
     io::Printer* printer) {
-  // TODO(jonskeet): Originally, this checked for Count > 0 first.
-  // The Write* call should make that cheap though - no need to generate it every time.
-  if (descriptor_->is_packed()) {
-    printer->Print(variables_,
-      "output.WritePacked$capitalized_type_name$Array($number$, $name$_);\n");
-  } else {
-    printer->Print(variables_,
-      "output.Write$capitalized_type_name$Array($number$, $name$_);\n");
-  }
+  printer->Print(
+    variables_,
+    "if ($name$_.Count > 0) {\n"
+    "  output.Write$packed$$capitalized_type_name$Array($number$, $name$_);\n"
+    "}\n");
 }
 
 void RepeatedPrimitiveFieldGenerator::GenerateSerializedSizeCode(