Preserve user group id when using /V1/customers/:customerId (PUT)

[bydrei]

Description

When you call /V1/customers/:customerId (PUT) and the customer has a group id already and you don't provide groupId in the request the customer group id is set to 1.
This fix preserves the group id.

Fixed Issues (if relevant)

1. Updating Customer through rest/all/V1/customers/:id resets group_id if group_id not passed in payload #14663

Manual testing scenarios

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.

[VladimirZaets]

Hi, @ferrazzuk , I made some changes in your code. I replace condition is_null to strict chek === null.
Thank you for collaboration.

[bydrei]

@VladimirZaets awesome, happy to collaborate.

[VladimirZaets]

Hi @ferrazzuk , please, sign CLA, otherwise, we can't process your pull request.
Your commit hasn't email setting. What would we process your pull request you should add email to your git settings and make a new commit instead of previous.

[bydrei]

@VladimirZaets done

[VladimirZaets]

Hi @ferrazzuk , you add new one commit, but the previous commit still exists. We can't merge PR with that commit. You should revert your changes to commit that is before your first commit (052b00119775fdef22a2527a5a53bdb112fd5dda) and make force push with your changes. Thus your first commit will be overridden.

[bydrei]

@VladimirZaets done

[VladimirZaets]

Hi @ferrazzuk , thank you for collaboration.
The Pull Request will be merged soon.

[magento-engcom-team]

Hi @ferrazzuk. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.1.15 release.

[bydrei]

@magento-engcom-team my pleasure :)