Fix panic on empty maintenance window (#1507)
Fix panic on empty maintenance window (#1507)

* fix maintenance window panic

* tiny cleanup

hashicorp/terraform-provider-google,hashicorp/terraform-provider-google,rosbo/terraform-provider-google,terraform-providers/terraform-provider-google,terraform-providers/terraform-provider-google,rosbo/terraform-provider-google,hashicorp/terraform-provider-google-beta,hashicorp/terraform-provider-google-beta,terraform-providers/terraform-provider-google,drebes/terraform-provider-google,ubschmidt2/terraform-provider-google,hashicorp/terraform-provider-google-beta,terraform-providers/terraform-provider-google,rosbo/terraform-provider-google,drebes/terraform-provider-google,hashicorp/terraform-provider-google,rosbo/terraform-provider-google,ubschmidt2/terraform-provider-google,drebes/terraform-provider-google,ubschmidt2/terraform-provider-google,hashicorp/terraform-provider-google,hashicorp/terraform-provider-google-beta,terraform-providers/terraform-provider-google,ubschmidt2/terraform-provider-google,drebes/terraform-provider-google