add try to default_letter_supplier

Author: alexnuttall

infrastructure/terraform/modules/backend-api/module_lambda_send_letter_proof.tf

@@ -17,7 +17,7 @@ module "lambda_send_letter_proof" {
   environment_variables = {
     CREDENTIALS_TTL_MS      = 900 * 1000
     CSI                     = local.csi
-    DEFAULT_LETTER_SUPPLIER = local.default_letter_supplier.name
+    DEFAULT_LETTER_SUPPLIER = try(local.default_letter_supplier.name, "unset")
     ENVIRONMENT             = var.environment
     INTERNAL_BUCKET_NAME    = module.s3bucket_internal.id
     NODE_OPTIONS            = "--enable-source-maps",