More OSS QUICHE build fixes.

Use `testing::IsNull()` instead of `nullptr` in `EXPECT_CALL` with callbacks,
similar to an earlier fix.

Also clean up some warnings by adding some `override`s in places.

PiperOrigin-RevId: 983330711

quiche/quic/moqt/moqt_session.h

diff --git a/quiche/quic/moqt/moqt_session.h b/quiche/quic/moqt/moqt_session.h
index a91ae90..5320b39 100644
--- a/quiche/quic/moqt/moqt_session.h
+++ b/quiche/quic/moqt/moqt_session.h
@@ -176,13 +176,13 @@
   }
 
   // SessionToUniStreamInterface implementation.
-  bool deliver_partial_objects() const {
+  bool deliver_partial_objects() const override {
     return parameters_.deliver_partial_objects;
   }
   // Called when the incoming track is malformed per Section 2.5 of
   // draft-ietf-moqt-moq-transport-12. Unsubscribe and notify the application so
   // the error can be propagated downstream, if necessary.
-  void OnMalformedTrack(ObjectSubscriber* track);
+  void OnMalformedTrack(ObjectSubscriber* track) override;
   quiche::QuicheWeakPtr<ObjectSubscriber> GetSubscribe(
       uint64_t track_alias) override {
     ObjectSubscriber* track = SubscribeByAlias(track_alias);
@@ -191,7 +191,8 @@
     }
     return track->weak_ptr();
   }
-  quiche::QuicheWeakPtr<ObjectSubscriber> GetFetch(uint64_t request_id) {
+  quiche::QuicheWeakPtr<ObjectSubscriber> GetFetch(
+      uint64_t request_id) override {
     auto it = fetch_by_id_.find(request_id);
     if (it == fetch_by_id_.end()) {
       return quiche::QuicheWeakPtr<ObjectSubscriber>();

quiche/quic/moqt/moqt_uni_stream_test.cc

diff --git a/quiche/quic/moqt/moqt_uni_stream_test.cc b/quiche/quic/moqt/moqt_uni_stream_test.cc
index 38a938b..0cfe405 100644
--- a/quiche/quic/moqt/moqt_uni_stream_test.cc
+++ b/quiche/quic/moqt/moqt_uni_stream_test.cc
@@ -42,6 +42,7 @@
 
 namespace {
 
+using ::testing::IsNull;
 using ::testing::Optional;
 using ::testing::Return;
 using ::testing::ReturnRef;
@@ -704,8 +705,8 @@
   // Cleanup
   EXPECT_CALL(mock_fetch_task_,
               OnStreamAndFetchClosed(absl::CancelledError("stream destroyed")));
-  EXPECT_CALL(mock_fetch_task_, set_task_destroyed_callback(nullptr));
-  EXPECT_CALL(mock_fetch_task_, set_can_read_callback(nullptr));
+  EXPECT_CALL(mock_fetch_task_, set_task_destroyed_callback(IsNull()));
+  EXPECT_CALL(mock_fetch_task_, set_can_read_callback(IsNull()));
   EXPECT_CALL(mock_control_stream_,
               OnStreamClosed(absl::CancelledError("stream destroyed"),
                              std::optional<DataStreamIndex>()));

quiche/web_transport/test_tools/in_memory_stream.h

diff --git a/quiche/web_transport/test_tools/in_memory_stream.h b/quiche/web_transport/test_tools/in_memory_stream.h
index f22b9f9..a134494 100644
--- a/quiche/web_transport/test_tools/in_memory_stream.h
+++ b/quiche/web_transport/test_tools/in_memory_stream.h
@@ -113,7 +113,7 @@
  public:
   using InMemoryStream::InMemoryStream;
 
-  void OnWrite(absl::string_view data) { write_buffer_.append(data); }
+  void OnWrite(absl::string_view data) override { write_buffer_.append(data); }
   absl::Status GetWriteStatus(bool is_write) const override {
     return absl::OkStatus();
   }