Save the price 0 as price in custom options [backport 2.2]

[raumatbel]

Related with the PR11842

Description

Possibility to save the price in custom options with value 0 or empty

Fixed Issues (if relevant)

1. magento/magento2#<4808>: The price of product custom option can't be set to 0

Steps to reproduce

1. Add an product custom option (Select Type)
2. Add new row and set price greater than 0.
3. Save product.
4. Change the row price to 0 or empty.
5. Save product.

Expected result

1. The row price becomes 0.

Actual result

1. The row price doesn't becomes 0.

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)