[datadog_synthetics_test] Handle secure config_variables on import

[skarimo]

On import, previous state does not exist so we cannot rely on the previous state to set the old values in state for example and pattern.

Stack trace:

➜  ~/Documents/terraform terraform plan -generate-config-out=generated3.tf
datadog_synthetics_test.test_integration_installation: Preparing import... [id=123]
datadog_synthetics_test.test_integration_installation: Refreshing state... [id=123]

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: Plugin did not respond
│ 
│ The plugin encountered an error, and failed to respond to the plugin.(*GRPCProvider).ReadResource call. The plugin logs may contain more details.
╵

Stack trace from the terraform-provider-datadog_v3.46.0 plugin:

panic: runtime error: index out of range [0] with length 0

goroutine 134 [running]:
github.com/terraform-providers/terraform-provider-datadog/datadog.buildTerraformConfigVariables({0x14000b3e140, 0x2, 0x9?}, {0x105113b40, 0x0, 0x0?})
        github.com/terraform-providers/terraform-provider-datadog/datadog/resource_datadog_synthetics_test_.go:2939 +0x5f0

[Drarig29]

Can you link to an issue or add the error message and stack trace to the PR's description?