Lenght of array[null] will return 1

Terraform
1

Lenght of array[null] will return 1

For example:

module foobar

resource "azurerm_lb" "lb" {
  ...
  # there will be 0 or 1
  count = 0
}

output "lb_ip" {
  value = [azurerm_lb.lb[0].private_ip_address]
}

main.tf

output "length" {
  value = length(module.foobar.lb_ip)
}

output length will return 1 ( it will count null as element )

I tried:
length(compact(module.foobar.lb_ip))
and
length([compact(module.foobar.lb_ip)])
even
length(concat(compact(module.foobar.lb_ip),[]))
, but witout success.

Any idea? Length should be 0 or 1 in case of azurerm_lb will be created.

Thanks

EDIT:

output "lb_ip" {
  value = concat([azurerm_lb.lb[0].private_ip_address],[])
}

will return:

b = [
  null,
]

output "lb_ip" {
  value = value = "${element(concat(azurerm_lb.lb.*.private_ip_address, list("")), 0)}"
}

will return:

b = ""

what is difference?
Will I be limited with this https://github.com/hashicorp/terraform/issues/17862 ?

Seems there is missing something like azurerm_lb.lb[0].private_ip_address is defined ? "foo" : "bar"