Fix Appending to Default in thrift::from_dynamic

Summary:
Related post: https://fb.workplace.com/groups/thriftusers/posts/1722521021790636

tl;dr

The `struct_with_list_field_with_default_values` test will fail, which is unexpected:
```
rdtscp@devvm34082•~/fbcode» hg diff thrift/lib/cpp2/folly_dynamic/internal/folly_dynamic-inl-post.h ; buck2 test thrift/lib/cpp2/folly_dynamic/test:folly_dynamic_test
 diff --git a/fbcode/thrift/lib/cpp2/folly_dynamic/internal/folly_dynamic-inl-post.h b/fbcode/thrift/lib/cpp2/folly_dynamic/internal/folly_dynamic-inl-post.h
 --- a/fbcode/thrift/lib/cpp2/folly_dynamic/internal/folly_dynamic-inl-post.h
+++ b/fbcode/thrift/lib/cpp2/folly_dynamic/internal/folly_dynamic-inl-post.h
@@ -163,7 +163,7 @@
     if (input.empty()) {
       return;
     }
-    out.clear();
+    // out.clear();
     do_reserve(out, input.size());
     for (const auto& i : input) {
       out.emplace_back();
✗ Fail: thrift/lib/cpp2/folly_dynamic/test:folly_dynamic_test - FromDynamic.struct_with_list_field_with_default_values (0.0s)
Note: Google Test filter = FromDynamic.struct_with_list_field_with_default_values
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from FromDynamic
[ RUN      ] FromDynamic.struct_with_list_field_with_default_values
fbcode/thrift/lib/cpp2/folly_dynamic/test/folly_dynamic_test.cpp:803: Failure
Expected equality of these values:
  res.list_field().value()
    Which is: { 1, 2, 3, 4, 5, 6, 7 }
  std::vector<int>({4, 5, 6, 7})
    Which is: { 4, 5, 6, 7 }

[  FAILED  ] FromDynamic.struct_with_list_field_with_default_values (1 ms)
[----------] 1 test from FromDynamic (1 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test suite ran. (1 ms total)
[  PASSED  ] 0 tests.
[  FAILED  ] 1 test, listed below:
[  FAILED  ] FromDynamic.struct_with_list_field_with_default_values

 1 FAILED TEST

Buck UI: https://www.internalfb.com/buck2/41d891b6-ef9b-4c45-802f-c650a296a203
Test UI: https://www.internalfb.com/intern/testinfra/testrun/14355223924851959
Network: Up: 0B  Down: 0B
Command: test.
Time elapsed: 3.0s
Test execution completed but the tests failed
Tests finished: Pass 23. Fail 1. Fatal 0. Skip 0. Build failure 0
1 TESTS FAILED
  ✗ thrift/lib/cpp2/folly_dynamic/test:folly_dynamic_test - FromDynamic.struct_with_list_field_with_default_values

Run $ fdb buck test <args> to debug thrift/lib/cpp2/folly_dynamic/test:folly_dynamic_test - FromDynamic.struct_with_list_field_with_default_values
      ^^^ just prefix your previous command! ($ fdb !!)
Learn more at https://fburl.com/fdb
Use Devmate to fix your errors. Just click the following link: fb-vscode://nuclide.code-compose/devmate/fixtests?run_id=14355223924851959
```

Reviewed By: Mizuchi, thedavekwon

Differential Revision: D82813298

fbshipit-source-id: 81864aafbe8743fccd966cd85ad4bd5ddcda9aac

Author: Alexander Wilson

thrift/lib/cpp2/folly_dynamic/internal/folly_dynamic-inl-post.h

@@ -163,6 +163,7 @@ struct dynamic_converter_impl<apache::thrift::type::list<VTag>> {
     if (input.empty()) {
       return;
     }
+    out.clear();
     do_reserve(out, input.size());
     for (const auto& i : input) {
       out.emplace_back();

thrift/lib/cpp2/folly_dynamic/test/folly_dynamic_test.cpp

@@ -793,3 +793,12 @@ TEST(FromDynamic, struct_with_vector_bool) {
       v, dynamic_format::PORTABLE, format_adherence::LENIENT);
   EXPECT_EQ(res.values(), std::vector<bool>({true, false, true, false}));
 }
+
+TEST(FromDynamic, struct_with_list_field_with_default_values) {
+  folly::dynamic v = folly::dynamic::object();
+  v["list_field"] = folly::dynamic::array(4, 5, 6, 7);
+  auto res = from_dynamic<
+      test_cpp2::cpp_reflection::StructWithListFieldWithDefaultValues>(
+      v, dynamic_format::PORTABLE, format_adherence::STRICT);
+  EXPECT_EQ(res.list_field().value(), std::vector<int>({4, 5, 6, 7}));
+}

thrift/test/reflection/reflection.thrift

@@ -675,3 +675,7 @@ struct StructWithAdaptedField {
 struct StructWithVectorBool {
   1: list<bool> values;
 }
+
+struct StructWithListFieldWithDefaultValues {
+  1: list<i32> list_field = [1, 2, 3];
+}