r/aws_ecs_service: fix crash from nil service_registries item

This appears only to be possible with dynamic blocks as explicitly setting a nil `service_registries` block will prompt for the required attributes within the block. However, this fix should prevent the nil value from being expanded and causing a crash under the dynamic block scenario.

```console
% make testacc PKG=ecs TESTS=TestAccECSService_ServiceRegistries
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.22.6 test ./internal/service/ecs/... -v -count 1 -parallel 20 -run='TestAccECSService_ServiceRegistries'  -timeout 360m

--- PASS: TestAccECSService_ServiceRegistries_basic (145.59s)
--- PASS: TestAccECSService_ServiceRegistries_container (145.60s)
--- PASS: TestAccECSService_ServiceRegistries_removal (156.46s)
--- PASS: TestAccECSService_ServiceRegistries_changes (279.51s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/ecs        285.700s
```

.changelog/38883.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+resource/aws_ecs_service: Fix crash from nil `service_registries` item
+```

internal/service/ecs/flex.go

@@ -164,6 +164,10 @@ func expandServiceRegistries(tfList []interface{}) []awstypes.ServiceRegistry {
  apiObjects := make([]awstypes.ServiceRegistry, 0, len(tfList))
 
  for _, tfMapRaw := range tfList {
+ if tfMapRaw == nil {
+ continue
+ }
+
  tfMap := tfMapRaw.(map[string]interface{})
  apiObject := awstypes.ServiceRegistry{
  RegistryArn: aws.String(tfMap["registry_arn"].(string)),