[party] Short circuit expensive loop at the end of every party (#37004)

Before:
```
----------------------------------------------------------------------------------------------------
Benchmark                                                          Time             CPU   Iterations
----------------------------------------------------------------------------------------------------
grpc_core::BM_UnaryWithSpawnPerEnd<CallSpineFixture>            1171 ns         1171 ns      2373821
grpc_core::BM_UnaryWithSpawnPerOp<CallSpineFixture>             1326 ns         1326 ns      2124026
grpc_core::BM_ClientToServerStreaming<CallSpineFixture>          256 ns          256 ns     10512990
grpc_core::BM_UnaryWithSpawnPerEnd<ForwardCallFixture>          2957 ns         2957 ns       949121
grpc_core::BM_UnaryWithSpawnPerOp<ForwardCallFixture>           3172 ns         3172 ns       882463
grpc_core::BM_ClientToServerStreaming<ForwardCallFixture>        500 ns          500 ns      5765914
```

After:
```
----------------------------------------------------------------------------------------------------
Benchmark                                                          Time             CPU   Iterations
----------------------------------------------------------------------------------------------------
grpc_core::BM_UnaryWithSpawnPerEnd<CallSpineFixture>            1147 ns         1147 ns      2451909
grpc_core::BM_UnaryWithSpawnPerOp<CallSpineFixture>             1325 ns         1325 ns      2107818
grpc_core::BM_ClientToServerStreaming<CallSpineFixture>          256 ns          256 ns     10896638
grpc_core::BM_UnaryWithSpawnPerEnd<ForwardCallFixture>          2876 ns         2876 ns       954360
grpc_core::BM_UnaryWithSpawnPerOp<ForwardCallFixture>           3148 ns         3148 ns       891892
grpc_core::BM_ClientToServerStreaming<ForwardCallFixture>        503 ns          503 ns      5518039
```

Closes #37004

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37004 from ctiller:cancel-fast 6b6b2b449c
PiperOrigin-RevId: 646138952

src/core/lib/promise/party.cc

@@ -178,6 +178,7 @@ Party::Participant::~Participant() {
 Party::~Party() {}
 
 void Party::CancelRemainingParticipants() {
+  if (!sync_.has_participants()) return;
   ScopedActivity activity(this);
   for (size_t i = 0; i < party_detail::kMaxParticipants; i++) {
     if (auto* p =

src/core/lib/promise/party.h

@@ -224,6 +224,10 @@ class PartySyncUsingAtomics {
     return iteration_.load(std::memory_order_relaxed);
   }
 
+  bool has_participants() const {
+    return (state_.load(std::memory_order_relaxed) & kAllocatedMask) != 0;
+  }
+
  private:
   bool UnreffedLast();