Fixed issue with empty scope being passed throwing exception

[sheridans]

Provide a narrative description of what you are trying to accomplish:

• Are you fixing a bug?
  • Detail how the bug is invoked currently.
  • Detail the original, incorrect behavior.
  • Detail the new, expected behavior.
  • Base your feature on the master branch, and submit against that branch.
  • Add a regression test that demonstrates the bug, and proves the fix.
  • Add a CHANGELOG.md entry for the fix.

When sending post request for authorisation with blank scope following error occurs:
TypeError: trim() expects parameter 1 to be string, null given in file /vendor/zendframework/zend-expressive-authentication-oauth2/src/Repository/Pdo/AbstractRepository.php on line 44

This pull request checks to see if the scope being passed is blank, returns an empty string and returns authorisation.

[ezimuel]

@sheridans thanks for your contribution. I just added a unit test for this PR. Thanks again!