azurerm_automation_software_update_configuration: set exists slice as nil when load SDK data to schema model

[wuxu92]

resolves: #21970

The API doesn't return the schedule block when the first version of software_update_configuration released, so I used a Decode to load the exists model in state, then cover with SDK data.

terraform-provider-azurerm/internal/services/automation/automation_software_update_configuration_resource.go

Lines 1026 to 1029 in f405d58

	var output SoftwareUpdateConfigurationModel
	if err := meta.Decode(&output); err != nil {
	return err
	}

It seems in some regions, like UK South, the API now returns the schedule info the GET request. we have to set the s.MonthlyOccurrence as nil before append the returned values into the schema model.

local test pass with no diff in second terraform plan

[jackofallops]

Thanks for this @wuxu92 - I've recently refactored this resource in #22204 and that PR will supersede his one, so I'm going to close this one out.

Thanks again.

[github-actions]

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