fix: fix the prefix input variable in the DA to allow null

solutions/fully-configurable/variables.tf

@@ -47,7 +47,7 @@ variable "prefix" {
 
   validation {
     # must not exceed 16 characters in length
-    condition     = length(var.prefix) <= 16
+    condition     = var.prefix == null || var.prefix == "" ? true : length(var.prefix) <= 16
     error_message = "Prefix must not exceed 16 characters."
   }
 }