Strings that change to empty fail to be updated

[peters95]

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Terraform Version
terraform 0.13.2

Affected Resource(s)
artifactory_remote_repository
Probably all resources that have string fields that it is valid to be the empty string.

Terraform Configuration Files
Using gocenter.io as a remote for the purposes of this example

Initial state

resource "artifactory_remote_repository" "someremote" {
url = "https://gocenter.io"
key = "gocenter"
package_type = "go"
repo_layout_ref = "go-default"
proxy = "someproxy"
}
Second apply

resource "artifactory_remote_repository" "someremote" {
url = "https://gocenter.io"
key = "gocenter"
package_type = "go"
repo_layout_ref = "go-default"
proxy = ""
}
Expected Behavior
Proxy should have been removed with a change of

proxy "someproxy" -> ""

Actual Behavior
Proxy field not passed to the API, as it got reduced to null

proxy "someproxy" => null

This retains the value "someproxy" in the remote.

Steps to Reproduce
This affects any string field in which changing the field to "" in the API would be a valid change.

terraform apply with initial state
terraform apply after changing a string field to ""
Possible fix
At https://github.com/atlassian/terraform-provider-artifactory/blob/master/pkg/artifactory/util.go#L15
d.GetOk(key) should perhaps be d.GetOkExists(key) so that empty strings are not treated as null

[chb0github]

copy of #2