Fixed plugin generation sort order requirement

[drpayyne]

Description

This PR

• makes the requirement of the sortOrder field in the 'create a new plugin' generation dialog, optional.
• sets the default value as empty as recommended in the reported issue.
• generates the sortOrder attribute in the di.xml plugin declaration only if sort order was initially set and no empty.
• adds test case for scenario with no empty sort order field (sort order attribute should not be generated in XML file)

Fixed Issues

1. Fixes Create Plugin action should default to no sortOrder #385: Create Plugin action should default to no sortOrder

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 integration/functional tests (if applicable)
• All automated tests passed successfully (all builds are green)

[coderimus]

Hello @drpayyne
Thank you for your contribution. According to the points policy, your contribution can be awarded with the next list of awards:

• test coverage - Test coverage - Contribution contains a fix or improvement and new tests or test cases. -> here you change the tests
• bug fix - Contribution fixes one or more known GitHub issues. -> you fixed the incorrect plugin sort order according to the report
  Thank you!