Code more defensively when accessing current_user()

[fozboz]

The current_user() method returns null if the user is anonymous. With the previous implementation, one would see frequent warnings such as this in PHP logs.

PHP Warning: Attempt to read property "role" on null

This update adds checks to ensure that the current user is not null before attempting to access its properties.

I didn't update the instances of this method's use in views/admin/mirador/annotator.php because I figured it should be impossible to reach that code if you are not logged in.

My editor also removed all trailing whitespace.

[dicksonlaw583]

Thank you for the PR. Going forward, instead of fetching roles from current_user(), I would prefer to migrate the permissions to an ACL-based setup. Example from a recent retrofit on Job Diagnostics:

• Main hook: https://github.com/utlib/JobDiagnostics/blob/master/JobDiagnosticsPlugin.php#L95
• Role check: https://github.com/utlib/JobDiagnostics/blob/master/controllers/TestsController.php#L55