Update postgres_version to use environment var is present

What does this MR do?

Defines postgres_version to check for the value of the environment variable `POSTGRES_VERSION` if not found the var will be blank. Also updates checks for postgres_version is defined to instead check for a value.

## Related issues

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
  • Code:
    • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
    • Documentation created/updated in the same MR.
    • If this MR adds an optional configuration - check that all permutations continue to work.
    • For Terraform changes: set up a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Self-Managed Platform team if unsure.