Handle multipart MIME parsing with no parts

pkg/http_server/test/http_multipart_test.dart

Index: pkg/http_server/test/http_multipart_test.dart
diff --git a/pkg/http_server/test/http_multipart_test.dart b/pkg/http_server/test/http_multipart_test.dart
index d1d08434fcaa0c915e93550c34d42dce9e5ecb28..f7e5cbb4c9a7abd2f9c926055ca5e207b745b430 100644
--- a/pkg/http_server/test/http_multipart_test.dart
+++ b/pkg/http_server/test/http_multipart_test.dart
@@ -93,6 +93,16 @@ void postDataTest(List<int> message,
   });
 }
 
+void testEmptyPostData() {
+  var message = '''
+------WebKitFormBoundaryU3FBruSkJKG0Yor1--\r\n''';
+
+  postDataTest(message.codeUnits,
+               'multipart/form-data',
+               '----WebKitFormBoundaryU3FBruSkJKG0Yor1',
+               []);
+}
+
 void testPostData() {
   var message = '''
 \r\n--AaB03x\r
@@ -212,5 +222,6 @@ Content of file\r
 
 
 void main() {
+  testEmptyPostData();
   testPostData();
 }