[synthetics_test] Handle request_file when state does not exist

[skarimo]

During imports or manual edits of the synthetics test, the request_file content might not be in the state. To avoid panics, check if the map or map item is nil first.

Example stack trace:

Stack trace from the terraform-provider-datadog plugin:

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

goroutine 146 [running]:
github.com/terraform-providers/terraform-provider-datadog/datadog.buildTerraformBodyFiles(0x1400041a0a0, {0x102f02d20, 0x0, 0x1?})
	/dd/terraform-provider-datadog/datadog/resource_datadog_synthetics_test_.go:2882 +0x3bc
github.com/terraform-providers/terraform-provider-datadog/datadog.updateSyntheticsAPITestLocalState(0x14000bc4100, 0x1400041ab40)
	/dd/terraform-provider-datadog/datadog/resource_datadog_synthetics_test_.go:1867 +0x2264
github.com/terraform-providers/terraform-provider-datadog/datadog.resourceDatadogSyntheticsTestRead({0x10204e698?, 0x140008cb050?}, 0x14000bc4100, {0x101be8800?, 0x14000868750?})
	/dd/terraform-provider-datadog/datadog/resource_datadog_synthetics_test_.go:1470 +0x7e4
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).read(0x14000341ce0, {0x10204e698, 0x140008cb050}, 0x14000bc4100, {0x101be8800, 0x14000868750})