Avoid notices if file does not exist

1. I've got notices on files
my-theme/woocommerce/single-product\add-to-cart\bundle.php, 
my-theme/woocommerce/single-product\bundled-item-image.php, 
my-theme/woocommerce/single-product\bundled-item-title.php
which are Template Overrides (WooCommerce Product Bundles)
and do not exist in the WC plugin.

2. There should be a more elegant way, because this is just to shut up notices. No version comparison performed.
Merge pull request #6635 from tivnet/patch-2

Avoid notices if file does not exist