Fix fatal error when no product id passed to wc_get_product

[ilearnio]

Issue: #883

Ticket:

Slack thread:

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Description

Fixes fatal error that happens when navigating to a post of type "page" that has a shortcode [product_page id="ID"]. The error was happening because product ID was not detected for such a page and thus wc_get_product (that was called with no args) couldn't detect the product automatically, which caused error in intenal WC code.

Steps to test:

1. Add a product to home page with a shortcode [product_page id="ID"]
2. Navigate to home page

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Changelog entry

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Closes #883 .