Product notifications

when I set up product notifications the email that is received by the customer is blank. Any ideas why this would be happening?

Any number of potential reasons.

Would check the email system for general problems then also would consider what was done to setup for product notification. (most likely reason of issue, but the thread is pretty bare on the status of being able to send an email that is received in its entirety).
Notifications: https://www.zen-cart.com/content.php?220-how-does-the-product-notification-system-work
Email: https://www.zen-cart.com/content.php?151-emails-are-not-received-or-sending
Email: https://www.zen-cart.com/content.php?61-my-emails-are-not-arriving-to-all-domains-or-rejected-from-nobody

Notification SQL that I had found back in 2013 and copied to a store to make it "easier" to reproduce:

INSERT INTO query_builder (`query_id`, `query_category`, `query_name`, `query_description`, `query_string`, `query_keys_list`) VALUES ('', 'email,newsletters', 'All customers who have NOT purchased product #196', 'All customers who have NEVER purchased product #196, ignoring newsletter-subscription status.', 'SELECT c.customers_email_address, c.customers_lastname, c.customers_firstname from TABLE_CUSTOMERS c WHERE c.customers_id NOT IN (SELECT c.customers_id FROM TABLE_CUSTOMERS c, TABLE_ORDERS o, TABLE_ORDERS_PRODUCTS op WHERE c.customers_id=o.customers_id AND o.orders_id=op.orders_id AND op.products_id=196) GROUP BY c.customers_email_address ORDER BY c.customers_lastname, c.customers_firstname ASC', ''); 

The above query (starting with SELECT in the middle of the INSERT INTO statement) returns the email address of all people that did not order the product number identified (196) in this case.

I would probably modify that statement a little in today's environment to remove the first parameter of both sets within parentheses (ie. The query_id and the associated double single quotes)