chore: set github enterprise default to null (see reason in PR) (#30)

When I set it, I get the message that this integration isn't available.
I believe this is available only to certain Spacelift situations when
using the GitHub custom app integration, so I don't think this is most
users?


![image](https://github.com/user-attachments/assets/31d492ad-6abe-4db8-9d1e-ba91873d87a5)


According to documentation, this is only for custom GitHub app for the
integration, (hence it works for our own `mp-infra` since we use our own
Masterpoint app, but not others repos).

But because of this, I'm thinking we set this variable default to null.
Feel free to let me know if this isn't a good idea. Just a suggestion I
had while using this.

## References:

![image](https://github.com/user-attachments/assets/9fea998c-60d2-4701-b574-b6869858a00e)

https://registry.terraform.io/providers/spacelift-io/spacelift/latest/docs/resources/stack#github_enterprise-1



![image](https://github.com/user-attachments/assets/a2512d5f-6ea7-45ec-b7a2-fb3e5a59d194)

https://spacelift.io/blog/deploying-spacelift-with-spacelift

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

## Summary by CodeRabbit

- **New Features**
- Introduced a default value of `null` for the `github_enterprise`
variable, allowing for an explicit null state when no value is provided.

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

variables.tf

@@ -20,6 +20,7 @@ variable "github_enterprise" {
     id        = optional(string)
   })
   description = "The GitHub VCS settings"
+  default     = null
 }
 
 variable "repository" {