Return URL in getThumbnailUrl instead of nothing

[samgranger]

In Magento/Catalog/Helper/Product the getThumbnailUrl method currently only returns an empty string, this should, of course, return the Product Thumbnail Url.

Fixed Issues (if relevant)

n/a

Manual testing scenarios

$this->productHelper->getThumbnailUrl($product);

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)