Connection config: tweak reading of sensitive fields, reuse local value of a sensitive field even if API returned null

[fivetran-renat-sh]

I) Fix for empty config section after importing fivetran_connector resource.

1. Given TF config

resource "fivetran_connector" "connector_1" { 
}

2. terraform import fivetran_connector.connector_1 "connector_id1"
3. config section is empty after import:

resource "fivetran_connector" "connector_1" { 
    group_id             = "group_id1"
    ...
    config {
    }
}

II) Fix for a scenario:

1. TF config with a connection with sensitive field like

resource "fivetran_connector" "test_connector" {
    ...
    config {
         sensitive_field = "secret value"
         ...
    }
}

2. Connection's config was updated via UI - value of this sensitive value was removed.
3. GET /connections/{} response now doesn't include this field:

       ...
		"config": {
			"schema": "schema1",
			"another_field": "value1"
		}
      }

5. TF config was changed
6. terrafrom apply fails with

        Error: Provider produced inconsistent result after apply
        
        When applying changes to fivetran_connector.test_connector, provider
        "provider[\"registry.terraform.io/hashicorp/fivetran-provider\"]" produced an
        unexpected new value: .config.sensitive_field: inconsistent values for
        sensitive attribute.
        
        This is a bug in the provider, which should be reported in the provider's own
        issue tracker.