fix: Render empty subscription arrays correctly (#4195)

[Omansh5]

Overview/Summary

This PR fixes Bug @#4195(Azure/Azure-Landing-Zones#4195) by correctly rendering empty subscription arrays for optional platform subscriptions in the generated Bicep parameter files.

Previously, when the Identity or Security subscription IDs were left empty, the generated .bicepparam files contained:

subscriptionsToPlaceInManagementGroup: ['']

This PR updates the rendering logic so that empty subscription values are rendered as:

subscriptionsToPlaceInManagementGroup: []

while preserving valid subscription IDs.

This PR fixes/adds/changes/removes

1. Updates the Bicep file rendering logic in modules/file_manipulation/locals.bicep.tf.
2. Converts subscriptionsToPlaceInManagementGroup: [''] to subscriptionsToPlaceInManagementGroup: [] after template rendering.
3. Preserves valid subscription IDs and arrays.
4. Scopes the replacement specifically to subscriptionsToPlaceInManagementGroup so unrelated [''] values are not modified.

Validation

Validation was performed using the ALZ Accelerator with the following configuration:

• • IaC: Bicep
• Management subscription: Configured
• Connectivity subscription: Configured
• Identity and Security subscriptions: Tested with both empty and individually empty configurations

Verified that:

• Both Identity and Security empty → subscriptionsToPlaceInManagementGroup: [].
• Only Identity empty → empty subscription array is rendered as [].
• Only Security empty → empty subscription array is rendered as [].
• Valid Management and Connectivity subscription IDs are preserved.
• Multiple valid subscription IDs are preserved unchanged.
• An unrelated [''] value remains unchanged.

The final rendering therefore removes the invalid empty subscription entry only from the affected parameter while preserving unrelated values.

Fixes #4195

[Copilot]

Pull request overview

This PR addresses Bug #4195 by adjusting the Bicep .bicepparam templating pipeline to render empty subscription arrays as [] (instead of ['']) after template rendering, while leaving populated subscription arrays unchanged.

Changes:

• Post-processes rendered .bicepparam content to convert [''] to [].
• Keeps the existing escaping/unescaping flow around templatestring() intact.

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