[BUGFIX] GITHUB-18264 Backport of #17799 for the 2.2 branch

[kanduvisla]

Description

This PR is a backport of #17799 for the 2.2 branch. It fixes issue #18264 where the price indexer would generate unwanted results if a special price date is set, but the actual special price is omitted.

Fixed Issues (if relevant)

1. M2.2.6 : "Order by price" not working in product listing #18264

Manual testing scenarios

1. Create some products with different prices.
2. Give them no special price (empty field), but give them a special price from date in the past and/or a special price to date in the future.
3. Run the price reindexer.
4. The products that have special price set to (empty field), are now set to the regular price as min_price, resulting in a correct working price sorting.

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-cicd2]

All committers have signed the CLA.

[magento-engcom-team]

Hi @kanduvisla. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me $VERSION instance - deploy vanilla Magento instance

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[sidolov]

Hi @kanduvisla , looks like you made some commits with email different than in your GitHub profile, please, add email from commits to your profile and sign CLA!
Thank you!

[kanduvisla]

@sidolov : I added the e-mail address to my account on GitHub in re-signed the CLA

[magento-engcom-team]

Hi @sidolov, thank you for the review.
ENGCOM-3261 has been created to process this Pull Request

[magento-engcom-team]

@kanduvisla thank you for contributing. Please accept Community Contributors team invitation here to gain extended permissions for this repository.

[magento-engcom-team]

Hi @kanduvisla. Thank you for your contribution.
We will aim to release these changes as part of 2.2.8.
Please check the release notes for final confirmation.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.