Bug: Adding conditional statements for subnet references

[oZakari]

Overview/Summary

Fixes #746 and also addresses related similar scenario if a customer removed the GatewaySubnet object from parSubnets by adding conditionals to the resource references for those subnets.

This PR fixes/adds/changes/removes

1. Add conditional statement for existing resource reference for GatewaySubnet
2. Add conditional statement for existing resource reference for FirewallSubnet

Breaking Changes

1. None

Testing Evidence

Ran a deployment with parExpressRouteGatewayEnabled, parVpnGatewayEnabled, and parAzFirewallName set to false as well as their parSubnet objects removed.

As part of this Pull Request I have

• Read the Contribution Guide and ensured this PR is compliant with the guide
• Ensured the resource API versions in .bicep file/s I am adding/editing are using the latest API version possible
• Checked for duplicate Pull Requests
• Associated it with relevant GitHub Issues
• (ALZ Bicep Core Team Only) Associated it with relevant ADO Items
• Ensured my code/branch is up-to-date with the latest changes in the main branch
• Performed testing and provided evidence.
• Updated one or more of the following tests (if required)
  • Unit
  • Linting
  • E2E (End-To-End)
  • ValidateAzCloud (Base validation in Azure Cloud)
  • ValidateMcCloud (Base validation in Azure China Cloud)
• Updated relevant and associated documentation (e.g. Contribution Guide, Module READMEs, Wiki Docs etc.)
• If relevant, created or updated Code Tours here