Specify all variables in the basic_usage example as non-nullable

In #221 we made all of the variables in the example module non-nullable
but did not do the same to the `basic_usage` example. Since it uses
variables beyond the requirements of the example module it makes sense
to also specify the example's variables as non-nullable.

examples/basic_usage/variables.tf

@@ -6,6 +6,7 @@
 
 variable "tf_role_arn" {
   description = "The ARN of the role that can terraform non-specialized resources."
+  nullable    = false
   type        = string
 }
 
@@ -18,18 +19,21 @@ variable "tf_role_arn" {
 variable "ami_owner_account_id" {
   default     = "self"
   description = "The ID of the AWS account that owns the AMI, or \"self\" if the AMI is owned by the same account as the provisioner."
+  nullable    = false
   type        = string
 }
 
 variable "aws_availability_zone" {
   default     = "a"
   description = "The AWS availability zone to deploy into (e.g. a, b, c, etc.)."
+  nullable    = false
   type        = string
 }
 
 variable "aws_region" {
   default     = "us-east-1"
   description = "The AWS region to deploy into (e.g. us-east-1)."
+  nullable    = false
   type        = string
 }
 
@@ -38,5 +42,6 @@ variable "tags" {
     Testing = true
   }
   description = "Tags to apply to all AWS resources created."
+  nullable    = false
   type        = map(string)
 }