Change std::nullopt to absl::nullopt in QuicStreamPriority

Using std::nullopt causes the following error while rolling quiche to Chromium: "error: no viable conversion from returned value of type 'const nullopt_t' to function return type 'absl::optional<QuicStreamPriority>'". Hence, this CL changes the std::nullopt to a absl::nullopt.

Also, adding a check to prevent this from happening again.

PiperOrigin-RevId: 495735836

quiche/quic/core/quic_stream_priority.cc

diff --git a/quiche/quic/core/quic_stream_priority.cc b/quiche/quic/core/quic_stream_priority.cc
index 1209258..c378bfa 100644
--- a/quiche/quic/core/quic_stream_priority.cc
+++ b/quiche/quic/core/quic_stream_priority.cc
@@ -43,7 +43,7 @@
   absl::optional<quiche::structured_headers::Dictionary> parsed_dictionary =
       quiche::structured_headers::ParseDictionary(priority_field_value);
   if (!parsed_dictionary.has_value()) {
-    return std::nullopt;
+    return absl::nullopt;
   }
 
   uint8_t urgency = QuicStreamPriority::kDefaultUrgency;