Add availability to leave empty config for events.xml

[lisovyievhenii]

Description (*)

The node in events.xml file cannot have no any children nodes.

Fixed Issue

#15931: events.xml cant have no childrens, others can [Magento 2.2.4]

Manual testing scenarios (*)

1. Create new module
2. Create events.xml
3. Fill with only base node "" without any children
4. Run php bin.magento cache:flush

Expected result

Upgrade + Flush + Clean should run fine trough, page is loading afterwards in developer mode

Actual result

Cache Flush triggers the error:

  [Magento\Framework\Exception\LocalizedException]                               
  Invalid XML in file /var/www/html/app/code/Vendor/Modulename/etc/events.xml:  
  Element 'config': Missing child element(s). Expected is ( event ).             
  Line: 2

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)

Backport for: #19146

[magento-engcom-team]

Hi @lisovyievhenii. Thank you for your contribution
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[lisovyievhenii]

The branch name was incorrectly named. Don't pay attention on it

[arnoudhgz]

@magento-engcom-team please consider that this will make the code less strict. I already mentioned that in the issue that it is not really an issue. To me it makes no sense to make the code less strict, just for debugging.

[lisovyievhenii]

@arnoudhgz all other xsd schemas support an empty configuration. So it makes sense to add this fix into events.xsd schema, to make it similar to others

[arnoudhgz]

@lisovyievhenii I think the others should be made more strict...

But it is not my call, I just want it here as a side note for the reviewer, since Magento never replied on my issues with this in the issue.

Sorry, accidentally clicked the close button, reopened again 👍

[lisovyievhenii]

Could you please re-run tests or say why it was failed?

[rogyar]

I'm going to re-run the tests internally.

[magento-engcom-team]

Hi @lisovyievhenii. 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.