fix: Fix zone auto-detection when var.zones are not set

[legal90]

In v17.0.0 (#507) there was an error in the detection of the default value for local.zone, which takes place when var.zone is not set:

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: Invalid index
│
│   on .terraform/modules/postgresql/modules/postgresql/read_replica.tf line 22, in locals:
│   22:   zone = var.zone == null ? data.google_compute_zones.available[0].names[0] : var.zone
│     ├────────────────
│     │ data.google_compute_zones.available is empty tuple
│
│ The given key does not identify an element in this collection value: the
│ collection has no elements.

This PR fixes this issue.

Solution details

In line 21 we see:

locals {
  # ...
  // Zone for replica instances
  zone = var.zone == null ? data.google_compute_zones.available[0].names[0] : var.zone
}

It means that we are going to rely on data.google_compute_zones only when var.zone == null. So that, we should flip the "count" condition for data.google_compute_zones, as I did in this PR.

After that fix terraform plan works as expected with both cases: when var.zone is defined and when when it's not defined (null).

cc @imrannayer , @g-awmalik

[imrannayer]

/gcbrun

[imrannayer]

/gcbrun

[imrannayer]

/gcbrun