azurerm_linux_function_app_slot - fixed panic when planning from a version older than 3.88.0

[christian-calabrese]

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Description

When planning changes on a linux function app slot that has been previously created with an older version of the provider, the plugin panics during the migration of the model.
This prevents from changes in the whole terraform workspace, so may be blocking for the many users in the same situation.

Stack trace from the terraform-provider-azurerm_v3.101.0_x5 plugin:

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

goroutine 57 [running]:
github.com/hashicorp/terraform-provider-azurerm/internal/services/appservice/migration.(*LinuxFunctionAppSlotV0toV1).UpgradeFunc.LinuxFunctionAppSlotV0toV1.UpgradeFunc.func1({0x0?, 0x0?}, 0x0?, {0x0?, 0x0?})
...
panic: interface conversion: interface {} is nil, not string

The error is caused by unsafe get of the service_plan_id key from the rawState map during state migration from v0 to v1.

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Changes to existing Resource / Data Source

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The test has been taken creating the resource with the 3.87.0 version of the provider and planned again with 3.101.0 version. No plan edits returned.

Change Log

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

• azurerm_linux_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 #0000

Note

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

[mbfrahry]

LGTM!