Supports passing in compute_config = null

The logic around enabling Auto Mode is based on the value never being
null, so use `nullable = false` in the variable definition so that a
null value will be interpreted as nn empty map (the default value).

Fixes #3307

Author: lorengordon

variables.tf

@@ -47,6 +47,7 @@ variable "authentication_mode" {
 variable "cluster_compute_config" {
   description = "Configuration block for the cluster compute configuration"
   type        = any
+  nullable    = false
   default     = {}
 }