Make all fails when no apps configured and no serve config block

[jjgriff93]

Describe the bug
It should be possible to deploy FlowEHR without any serve configuration as it is optional and only required when deploying apps. Currently, if no serve config is provided you get the following error:

  │ Error: "app_auth.0.id": required field is not set
  │ 
  │   with provider["registry.terraform.io/integrations/github"],
  │   on provider.tf line 23, in provider "github":
  │   23: provider "github" {
  │ 
  ╵
  ╷
  │ Error: "app_auth.0.installation_id": required field is not set
  │ 
  │   with provider["registry.terraform.io/integrations/github"],
  │   on provider.tf line 23, in provider "github":
  │   23: provider "github" {
  │ 
  ╵

Expected behavior
make all runs without error when no apps are configured and no serve block provided in config.

The fix
In apps/variables.tf, changing the default values from null to strings will fix this:

default = {
    github_owner               = ""
    github_app_id              = ""
    github_app_installation_id = ""
  }

[jjgriff93]

Fixed in #317