Use explicit nullable type in GitRepositoryProviderTest.

[whataboutpereira]

Fix deprecations in tests.

  1x: Shivas\VersioningBundle\Provider\exec(): Implicitly marking parameter $execOutput as nullable is deprecated, the explicit nullable type must be used instead
    1x in GitRepositoryProviderTest::setUpBeforeClass from Shivas\VersioningBundle\Tests\Provider

  1x: Shivas\VersioningBundle\Provider\exec(): Implicitly marking parameter $execExitCode as nullable is deprecated, the explicit nullable type must be used instead
    1x in GitRepositoryProviderTest::setUpBeforeClass from Shivas\VersioningBundle\Tests\Provider

[dontub]

Thanks @whataboutpereira!