fix: Ensure isra_tag_values can be tried before defaulting to cluster_name on Karpenter module (#2631)

Co-authored-by: Bryant Biggs <bryantbiggs@gmail.com>

modules/karpenter/variables.tf

@@ -89,7 +89,7 @@ variable "irsa_tag_key" {
 variable "irsa_tag_values" {
   description = "Tag values (`{key = value}`) applied to resources launched by Karpenter through the Karpenter provisioner. Defaults to cluster name when not set."
   type        = list(string)
-  default     = null
+  default     = []
 }
 
 variable "irsa_ssm_parameter_arns" {