fix: autoscaler_version can't be null

```
│   on .terraform/modules/worker_pool/autoscaler.tf line 27, in data "archive_file" "binary":
│   27:   output_path = "ec2-workerpool-autoscaler_${var.autoscaler_version}.zip"
│     ├────────────────
│     │ var.autoscaler_version is null
│ 
│ The expression result is null. Cannot include a null value in a string template.
```

variables.tf

@@ -146,9 +146,10 @@ variable "enable_autoscaling" {
 }
 
 variable "autoscaler_version" {
-  type        = string
   description = "Version of the autoscaler to deploy"
+  type        = string
   default     = "v0.3.0"
+  nullable    = false
 }
 
 variable "autoscaler_architecture" {