[Backport] Fix empty cart validation

[wojtekn]

Original Pull Request

#21295

Description

This PR prevents from running UpdateItemQty validation before Clear Shopping Cart action is triggered. In the other case validation breaks for use case in which product existing in cart went out of stock in the meantime.

Fixed Issues

1. Cart can't be emptied if any product is out of stock #21294

Manual testing scenarios

1. Add simple product in backend, set positive quantity and Stock Status = In Stock
2. Open another browser tab and access product in frontend
3. Add product to cart
4. Navigate to cart
5. Enable "Clear Shopping Cart" button by running this code in console:
   jQuery('.action.clear').show()
   It's hidden in CSS for some reason.
6. Back to product form in the backend
7. Set Stock Status for product to Out of Stock, save
8. Click "Clear Shopping Cart" button in the cart in frontend

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)

[magento-engcom-team]

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[wojtekn]

@magento-engcom-team give me 2.2-develop instance

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[wojtekn]

• @magento-engcom-team give me test instance

[magento-engcom-team]

Hi @wojtekn. Thank you for your request. I'm working on Magento instance for you

[magento-engcom-team]

Hi @wojtekn, here is your new Magento instance.
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[wojtekn]

I reproduced the issue on the vanilla instance and then confirmed that PR fixes it on the instance based on the PR.

[magento-engcom-team]

Hi @sivaschenko, thank you for the review.
ENGCOM-4479 has been created to process this Pull Request

[soleksii]

✔️ QA Passed

[ghost]

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