Only use 'certificates' by default if it exists

src/imapdomo.skel

@@ -160,7 +160,7 @@ main() {
     CfgDir="$IMAPDOMO_CFGDIR"
     LogDir="$IMAPDOMO_USER_CACHE/logs"
     HeaderDir="$IMAPDOMO_USER_CACHE/headers"
-    Certs="$IMAPDOMO_CFGDIR/certificates"
+    test -f "$IMAPDOMO_CFGDIR/certificates" && Certs="$IMAPDOMO_CFGDIR/certificates"
     NoLock=false
     Debug=false
     #shellcheck disable=SC2034