bugfix: null value can't pass schema validation

[ms-henglu]

The PR fixes the validation error like the following config:

/*
│ embedded schema validation failed: the argument "body" is invalid:
│ `properties.distribution` is invalid, expect `string` but got `<nil>`
│ `properties.infrastructure` is invalid, expect `string` but got `<nil>`
│  You can try to update `azapi` provider to the latest version or disable the validation using the feature flag `schema_validation_enabled = false` within the resource block
*/

resource "azapi_resource" "connectedCluster" {
  type = "Microsoft.Kubernetes/connectedClusters@2024-01-01"
  name      = var.aksArcName
  parent_id = var.resourceGroup.id

  body = {
    kind = "ProvisionedCluster"
    properties = {
      aadProfile = {
        adminGroupObjectIDs = flatten(var.rbacAdminGroupObjectIds)
        enableAzureRBAC     = true
        tenantID            = var.azureRBACTenantId
      }
      agentPublicKeyCertificate = "" 
      azureHybridBenefit        = null
      distribution              = null
      infrastructure            = null
      privateLinkState          = null
      provisioningState         = null
    }
  }

  identity {
    type = "SystemAssigned"
  }

  location = var.resourceGroup.location
  }

  timeouts {}
}