fix(snackbar): preserve default borderWidth in Get.snackbar

[Sami0402]

Description

This fixes a crash that occurs when Get.snackbar is called with borderColor but without borderWidth.

Reproduction

Get.snackbar(
  "Title",
  "Message",
  borderColor: Colors.grey,
);

Current behavior

A Null check operator used on a null value exception is thrown because Get.snackbar forwards a nullable borderWidth to GetSnackBar, overriding its constructor default (1.0).

Fix

Preserve the default borderWidth by forwarding borderWidth ?? 1.0 from the Get.snackbar wrapper.