Optional custom fields

[c0deaddict]

When using the terraform-phpipam-provider we get these errors:

Error: Error from API (200): No custom fields defined

  on network.tf line 17, in data "phpipam_subnet" "ams_public":
  17: data "phpipam_subnet" "ams_public" {

Looking at the problem I found that this SDK assumes that custom fields are defined in phpIPAM. Without any custom fields defined it throws an error.

This PR sets that right, it will suppress the error if no custom fields are defined. It is a bit of an awkward way to do this but I couldn't find a better way.

[pavel-z1]

Hi @c0deaddict , thank you.
PR is merged.