Fix/issue 36668: Shows warning only when the variation price is empty

[iyut]

Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

This PR is to fix issue for a warning that being displayed when user put 0 price for product variation.
To fix that, I changed the condition in sql to be like this :

SELECT count(post_id) FROM {$wpdb->postmeta}
WHERE post_id in (" . implode( ',', array_map( 'absint', $variation_ids ) ) . ")
AND ( meta_key='_subscription_sign_up_fee' OR meta_key='_price' )
AND meta_value >= 0
AND meta_value != ''

By doing this, it allows the user to set the price with 0.

Closes #36668 .

How to test the changes in this Pull Request:

Using the WooCommerce Testing Instructions Guide, include your detailed testing instructions:

1. Create a new variable product.

2. Set two new attributes for the product like the screenshot below :
   

3. Go to "Variations" tab and click "Generate variations".

4. Add the regular price for the first variant with 0 and the other variants with any number but more than 0.
   

5. Click "Save Changes" to save the variations.

6. Notice that no Warning being displayed.

7. Now, empty the regular price for the first variant and click "Save Changes".
   

8. Notice that the warning being displayed :
   

[mattsherman]

I'll review this

[mattsherman]

Nice work on this @iyut!

In addition to fixing the issue, the code is now much clearer, thanks to your naming of $invalid_variation_count to $valid_variation_count.

[mattsherman]

Thanks for fixing this up... it was so confusing that this was incorrectly named $invalid_variation_count previously! Most of my time reviewing this was just making sure that I wasn't missing something with this obvious of a misname! 😄

[iyut]

Hi @mattsherman , just wanted to say thanks for reviewing this PR 🙏