Rework for PR #16222 .

[phoenix128]

Description

When running 'app:config:dump', admin fields get disabled, but clicking save from backend will process them anyway.
This behaivour causes a validation check on empty fields since the admin form fields are disabled and thus validation errors in many cases.

This PR checks when the field is read only and skips its save.

This is a more complete rework for PR #16222 .

Manual testing scenarios

1. Go to Stores -> Configuration -> General -> Currency Setup
2. Setup Base Currency, Default Display Currency and Allowed Currencies to whatever you like
3. Save
4. Run bin/magento app:config:dump
5. Go back to admin and refresh the page (fields are disabled).
6. Click on save
7. As result no errors should be displayed

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]

Hi @phoenix128. 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

For more details, please, review the Magento Contributor Assistant documentation

[bmxmale]

I have checked your fix. It's working as expected. Thx for fast response 👍

[orlangur]

LGTM 👍

[magento-engcom-team]

Hi @phoenix128. Thank you for your contribution.
We will aim to release these changes as part of 2.2.6.
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.