fix: retention_days validation logic for backward compatibility

The retention_days_cross_valid validation was causing 'argument must not be null'
errors in Terraform 1.6.0-1.11.4 and OpenTofu 1.6.0-1.9.3 when both min_retention_days
and max_retention_days were null (valid for standard vaults).

Changed from logical OR to conditional expression to handle null values properly:
- Returns true if either value is null (valid for standard vaults)
- Only performs min <= max comparison when both values are provided
- Maintains existing validation for air-gapped vaults via airgapped_vault_requirements_met

This ensures backward compatibility with older Terraform/OpenTofu versions while
preserving functionality for newer versions.

Fixes #281

Co-authored-by: Luis M. Gallardo D. <lgallard@users.noreply.github.com>

Author: claude[bot]

main.tf

@@ -17,7 +17,7 @@ locals {
   airgapped_vault_requirements_met = var.vault_type != "logically_air_gapped" || (var.min_retention_days != null && var.max_retention_days != null)
 
   # Cross-validation for retention days (unified validation approach)
-  retention_days_cross_valid = (var.min_retention_days == null || var.max_retention_days == null) || var.min_retention_days <= var.max_retention_days
+  retention_days_cross_valid = (var.min_retention_days == null || var.max_retention_days == null) ? true : var.min_retention_days <= var.max_retention_days
 
   # Vault reference helpers (dynamic based on vault type)
   vault_name = local.should_create_standard_vault ? try(aws_backup_vault.ab_vault[0].name, null) : (