fix: Update lookup logic for Atlantis URL due to variable optional attributes

[bryantbiggs]

Description

• Update lookup logic for Atlantis URL due to variable optional attributes

Motivation and Context

• The current logic will resolve try(var.atlantis.fqdn, module.alb.route53_records["A"].fqdn, null), as null when not providing a fqdn because of the variable optional attributes (does not fall back to the next value on error, no error provided - just null). This coupled with the overall coalesce() means that the ALB DNS name is commonly exported instead of the URL using the Route53 record created by the module

Breaking Changes

• No

How Has This Been Tested?

• I have updated at least one of the examples/* to demonstrate and validate my change(s)
• I have tested and validated these changes using one or more of the provided examples/* projects

• I have executed pre-commit run -a on my pull request

[antonbabenko]

This PR is included in version 5.0.2 🎉