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.
[2.2] Fixed nonce check in form handler
Small codestyle fix to add_discount method.
Coupon helper.
Unit Tests: Added test_add_discount method.
Unit Tests: Only check for error notices as methods add success notices.
Unit Tests: updated product class doc.
Merge branch 'master' of github.com:woothemes/woocommerce
Allow to filter empty tax rate code
Unit Tests: add method test_add_discount_duplicate.
[2.2] fixed the value format in stock field with wc_stock_amount(), closes #6622
Merge pull request #6641 from coenjacobs/filter-empty-rate-code

Allow to filter empty tax rate code
Allow some limited html in text attributes

Fixes #6620. @claudiosmweb @barrykooij see any issues with adding this
one? Doesn’t appear to break anything…
Merge pull request #6635 from tivnet/patch-2

Avoid notices if file does not exist
Split up the product widget methods
[2.2] Remove strtolower and capitalize status names

Fixes #6624
Telangana state