Fix type error in ResultPager::fetch

[nunoplopes]

When using etags, the reply from github may be empty and the underlying APIs will return an empty string. We need to flip those to empty arrays.
e.g.:

$github_builder
->addPlugin(newHttp\Client\Common\Plugin\HeaderSetPlugin([
'If-None-Match' => $etag,
]));
$api = $github_client->user('user');
$paginator = new \Github\ResultPager($github_client);
$data = $paginator->fetch($api, 'events', [$username]);
// $data should be [] if $etag is the latest

[acrobat]

@nunoplopes thanks for the PR! Can you also add a test for this to avoid any regressions in the future? Thanks!

See: https://github.com/KnpLabs/php-github-api/blob/master/test/Github/Tests/ResultPagerTest.php

[nunoplopes]

added tests + redone the fix based on the fix committed for #1135

[acrobat]

Thank you @nunoplopes