[Fast Pair] Check for empty callback in gatt_service_client_impl

Prevent crash if |GattDiscoveryCompleteForService| gets called
more than once with a valid FP service.

BUG=b:300596153
TEST=Unit tests and manual test initial FP with Pixel Buds A and
JBL TUNE230NC TWS.

(cherry picked from commit b2dbd2d)

Change-Id: Ic1e7a5de3054ff540835b8431d27e6b7130408f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4873848
Commit-Queue: Katherine Lai <laikatherine@chromium.org>
Reviewed-by: Juliet Lévesque <julietlevesque@google.com>
Cr-Original-Commit-Position: refs/heads/main@{#1198599}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4889800
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Auto-Submit: Katherine Lai <laikatherine@chromium.org>
Commit-Queue: Juliet Lévesque <julietlevesque@google.com>
Cr-Commit-Position: refs/branch-heads/5993@{#782}
Cr-Branched-From: 5113507-refs/heads/main@{#1192594}

ash/quick_pair/fast_pair_handshake/fast_pair_gatt_service_client_impl.cc

@@ -524,7 +524,7 @@ void FastPairGattServiceClientImpl::GattDiscoveryCompleteForService(
 
     if (pair_failure.has_value()) {
       NotifyInitializedError(pair_failure.value());
-    } else {
+    } else if (on_initialized_callback_) {
       is_initialized_ = true;
       RecordGattInitializationStep(
           FastPairGattConnectionSteps::kConnectionEstablished);

ash/quick_pair/fast_pair_handshake/fast_pair_gatt_service_client_impl_unittest.cc

@@ -1291,5 +1291,15 @@ TEST_F(FastPairGattServiceClientTest, WriteEmptyPersonalizedName) {
   WritePersonalizedName(empty);
 }
 
+// Regression test for b/300596153
+TEST_F(FastPairGattServiceClientTest,
+       NoCrashWhenGattDiscoveryCompleteForServiceCalledTwice) {
+  SuccessfulGattConnectionSetUp();
+  NotifyGattDiscoveryCompleteForService(
+      ash::quick_pair::kFastPairBluetoothUuid);
+  NotifyGattDiscoveryCompleteForService(
+      ash::quick_pair::kFastPairBluetoothUuid);
+}
+
 }  // namespace quick_pair
 }  // namespace ash