clang-tidy: fix error in test_listener_filter.h (envoyproxy#38615)

This fixes the clang-tidy error in test_listener_filter.h as in "use '=
default' to define a trivial default constructor".

Contributes to envoyproxy#28566

Signed-off-by: Takeshi Yoneda <t.y.mathetake@gmail.com>

Author: mathetake

test/integration/filters/test_listener_filter.h

@@ -87,7 +87,7 @@ class TestUdpListenerFilter : public Network::UdpListenerReadFilter {
 
 class TestFirstPacketReceivedFilterState : public StreamInfo::FilterState::Object {
 public:
-  explicit TestFirstPacketReceivedFilterState() {}
+  explicit TestFirstPacketReceivedFilterState() = default;
   static const absl::string_view key() { return "test.filter_state.quic_first_packet_received"; }
   void incrementPacketCount() { packet_count_++; }
   void setPacketLength(size_t packet_length) { packet_length_ = packet_length; }