internal/generic: tweak check for user_agent via provider_meta

```console
% make testacc PKG_NAME=internal/aws/ec2 TESTARGS='-run=TestAccAWSEC2VPC_providerMeta'
TF_ACC=1 go test ./internal/aws/ec2 -v -count 1 -parallel 20 -run=TestAccAWSEC2VPC_providerMeta -timeout 180m

--- PASS: TestAccAWSEC2VPC_providerMeta (30.16s)
PASS
ok      github.com/hashicorp/terraform-provider-awscc/internal/aws/ec2  31.214s
```

```console
% make smoke
make: Running smoke tests...
make: NOTE: All tests should pass. Error output for sdk.proto, "Response contains error diagnostic" can be ignored.
TF_ACC=1 go test ./internal/aws/logs -v -count 1 -parallel 3 -run=TestAccAWSLogsLogGroup_\|TestAccAWSLogsLogGroupDataSource_ -timeout 180m

--- PASS: TestAccAWSLogsLogGroup_providerMeta (27.49s)
=== CONT  TestAccAWSLogsLogGroupDataSource_NonExistent
2025-12-11T10:01:30.174-0500 [ERROR] sdk.proto: Response contains error diagnostic: tf_req_id=058bc5d0-c157-9e24-4ac3-5b2eea9a6a18 tf_provider_addr=registry.terraform.io/hashicorp/awscc tf_data_source_type=awscc_logs_log_group tf_rpc=ReadDataSource tf_proto_version=6.10 diagnostic_detail="After attempting to read the data source, the API returned a resource not found error for the id provided. Original Error: couldn't find resource" diagnostic_severity=ERROR diagnostic_summary="AWS Data Source Not Found"
--- PASS: TestAccAWSLogsLogGroupDataSource_NonExistent (1.41s)
=== CONT  TestAccAWSLogsLogGroupDataSource_basic
--- PASS: TestAccAWSLogsLogGroup_basic (30.14s)
=== CONT  TestAccAWSLogsLogGroup_disappears
--- PASS: TestAccAWSLogsLogGroup_update (40.80s)
--- PASS: TestAccAWSLogsLogGroupDataSource_basic (24.79s)
--- PASS: TestAccAWSLogsLogGroup_disappears (29.87s)
PASS
ok      github.com/hashicorp/terraform-provider-awscc/internal/aws/logs 61.019s
```

Author: jar-b

internal/generic/resource.go

@@ -937,7 +937,7 @@ func (r *genericResource) bootstrapContextWithProviderMeta(ctx context.Context,
 	var metadata []string
 	d.Append(providerMeta.GetAttribute(ctx, path.Root("user_agent"), &metadata)...)
 
-	if metadata != nil {
+	if len(metadata) > 0 {
 		ctx = useragent.Context(ctx, useragent.FromSlice(metadata))
 	}