[BackPort] resolve typos and correct variable names

[viral-wagento]

Description

Fixed typos and added proper variable names in below files:
Original PR: #19288

app/code/Magento/Sales/Model/Order.php
app/code/Magento/Sales/etc/adminhtml/system.xml

Manual testing scenarios

1. app/code/Magento/Sales/Model/Order.php
   Line:668 - replaced $hasActinFlag to $hasActionFlag
   Line:688 - replaced $dueAmountCondition to $hasDueAmount
   Line:690 - replaced $creditmemos to $creditMemos

2. app/code/Magento/Sales/etc/adminhtml/system.xml
   Line:52 - replaced Allow Zero GrandTotal to Allow Zero GrandTotal for Credit Memos
   Line:54 - replaced Allow Zero GrandTotal for Creditmemo to Allow Zero GrandTotal for Credit Memos

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)

[sivaschenko]

Hi @viral-wagento thanks for your contribution, can you please forwardport this pull request to 2.3 as well.

Original pull request #16005

[VladimirZaets]

Hi @viral-wagento. Thanks for the collaboration. Please add the translation for "Allow Zero GrandTotal for Credit Memos" and "Allow Zero GrandTotal for Credit Memos" phrases to en_US.csv

[orlangur]

Unfortunately, we can't accept such Pull Requests to 2.2 release line, all code cleanup like typos, spelling mistakes, etc should be delivered to 2.3 release line. For more information, please, refer to Magento Code Contributions Guide.

[VladimirZaets]

Hi @viral-wagento. As @orlangur said above firstly PR should be delivered to 2.3 Magento branch. Please create PR to 2.3 Magento branch.

[orlangur]

@VladimirZaets it is already merged to 2.3 in #19288.

I would accept this one to 2.2 as it contains not just typo fixes but also missing translations.

[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-3677 has been created to process this Pull Request

[magento-engcom-team]

Hi @viral-wagento. 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.

[viral-wagento]

@magento-engcom-team @sidolov
This has already been developed and merged in 2.3 release line.
#19288
Thanks!!