fix: check if tfvars file exists only if when it's expected (#16)

## what

- If tfvars are not enabled, we should not check this file in
precondition.

## why

- Bug.

## references

- N/A


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **New Features**
- Enhanced flexibility in stack configuration by allowing stacks to
proceed without requiring a `.tfvars` file if not enabled.
- Improved handling of various stack configurations, including
`administrative`, `after_apply`, and `before_init`.

- **Bug Fixes**
- Refined conditions for the creation of stack resources based on
integration and drift detection settings.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

main.tf

@@ -292,7 +292,7 @@ resource "spacelift_stack" "default" {
   lifecycle {
     # Expected `tfvars` file exists
     precondition {
-      condition     = fileexists("${local.configs[each.key].project_root}/tfvars/${local.configs[each.key].tfvars_file_name}.tfvars")
+      condition     = try(local.configs[each.key].tfvars.enabled, true) ? fileexists("${local.configs[each.key].project_root}/tfvars/${local.configs[each.key].tfvars_file_name}.tfvars") : true
       error_message = <<-EOT
       The required .tfvars file is missing for stack "${each.key}".