fix: Ensure additional_thumbprint validation allows a null value (#26)

Thanks to @officel for input

Co-authored-by: NISHIMURA Yoshitaka <[email protected]>

variables.tf

@@ -18,7 +18,7 @@ variable "additional_thumbprints" {
   type        = list(string)
 
   validation {
-    condition     = length(var.additional_thumbprints) <= 4
+    condition     = var.additional_thumbprints == null ? true : length(var.additional_thumbprints) <= 4
     error_message = "Only 4 additional thumbprints can be set, for a maximum of 5 in the OIDC provider."
   }
 }