[Bug][Azure Firewall] Azure Firewall "additionalProperties" does not properly allow null values

additional properties should be "key": string? (optional string)

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/network/resource-manager/Microsoft.Network/stable/2022-05-01/azureFirewall.json#L620

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/network/resource-manager/Microsoft.Network/stable/2022-05-01/azureFirewall.json#L686

This prevents different sdks/cli/powershell from properly validating a value client side.

Open specification/network/resource-manager/Microsoft.Network/stable/2022-05-01/azureFirewall.json at the referenced lines 620 and 686 and inspect the Azure Firewall additional-properties definitions. Update the schema so the optional string value accepts null, then validate the specification using the repository's available checks; done means client SDK, CLI, and PowerShell validation can accept null.

Written by the indexing model from the issue text.