Fix patching in stateful disks1

[modular-magician]

Add possibility to remove Stateful Disks by using Null Values.
resolves hashicorp/terraform-provider-google#10266

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compute: added possibility to remove `stateful_disks` in `compute_instance_group_manager` and `compute_region_instance_group_manager`.

Derived from GoogleCloudPlatform/magic-modules#7254