Handle multipart MIME parsing with no parts

pkg/mime/test/mime_multipart_transformer_test.dart

Index: pkg/mime/test/mime_multipart_transformer_test.dart
diff --git a/pkg/mime/test/mime_multipart_transformer_test.dart b/pkg/mime/test/mime_multipart_transformer_test.dart
index 7bc2d7d928ba97afbd604f3a25c924a6fae540ac..0008ff01105f9aed9b4a2c4c8b50934e07e187cb 100644
--- a/pkg/mime/test/mime_multipart_transformer_test.dart
+++ b/pkg/mime/test/mime_multipart_transformer_test.dart
@@ -69,8 +69,12 @@ void _testParse(String message,
 }
 
 void _testParseValid() {
+  // Empty message from Chrome form post.
+  var message = '------WebKitFormBoundaryU3FBruSkJKG0Yor1--\r\n';
+  _testParse(message, "----WebKitFormBoundaryU3FBruSkJKG0Yor1", [], []);
+
   // Sample from Wikipedia.
-  var message = """
+  message = """
 This is a message with multiple parts in MIME format.\r
 --frontier\r
 Content-Type: text/plain\r