fix: Render empty subscription arrays correctly (#4195)

[Omansh5]

Overview/Summary

This PR fixes Bug #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 [''] to [] after template rendering.
3. Preserves valid subscription arrays for populated Management and Connectivity subscriptions.

Validation

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

• IaC: Bicep
• Identity subscription: Empty
• Security subscription: Empty
• Management subscription: Configured
• Connectivity subscription: Configured

Verified that:

• Identity renders:

  subscriptionsToPlaceInManagementGroup: []

• Security renders:

  subscriptionsToPlaceInManagementGroup: []

• Management renders the configured subscription ID.
• Connectivity renders the configured subscription ID.
• No occurrences of [''] remain in the generated local-output files.
• No unresolved template placeholders remain in the rendered .bicepparam files.

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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