azurerm_windows_function_app_slot - fixed panic when planning

[erjdev]

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Description

Similar to the issue fixed under the linux function app pull request #25838, we're seeing the same panic occurring for the Windows function app during the planning phase.

Stack trace from the terraform-provider-azurerm_v3.95.0_x5 plugin:

panic: interface conversion: interface {} is nil, not string

goroutine 57 [running]:
github.com/hashicorp/terraform-provider-azurerm/internal/services/appservice/migration.(*WindowsFunctionAppSlotV0toV1).UpgradeFunc.WindowsFunctionAppSlotV0toV1.UpgradeFunc.func1({0x0?, 0x0?}, 0x0?, {0x0?, 0x0?})
...
panic: interface conversion: interface {} is nil, not string

Just like the linux upgrader, the error is caused by the type assertion of service_plan_id; this PR adds the ok validity check on the assertion instead of allowing a fatal panic.

PR Checklist

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Changes to existing Resource / Data Source

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My test was failing to plan when using azurerm provider version 3.95, and now succeeds with this check during the migration.

Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• azurerm_windows_function_app_slot - fixed error when resource is firstly deployed with provider version <= 3.88.0 and then upgraded

This is a (please select all that apply):

• Bug Fix
• New Feature (ie adding a service, resource, or data source)
• Enhancement
• Breaking Change

Related Issue(s)

Fixes #24894

Note

If this PR changes meaningfully during the course of review please update the title and description as required.

[jackofallops]

Thanks @erjdev - LGTM 👍

[github-actions]

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