fix: Fix hubNetworking module output when DDoS is disabled

[cconstantin]

When disabling DDoS in hubNetworking the module fails because the DDoS plan is not actually provisioned and the output always requires it.

Overview/Summary

The DDoS plan id output is not conditional on whether DDoS is enabled, leading to failures.

Related Issues/Work Items

This PR fixes/adds/changes/removes

#860

Breaking Changes

Testing Evidence

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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

[oZakari]

/azp run validateazcloud

[oZakari]

Hi @cconstantin, thanks so much for submitting this PR to fix this!