Haskell: Highlight empty comments after 'import'

Summary:

In the following snippet,

    import Bar ()  --

the trailing, empty comment did not get highlighted.

Reviewers: dhaumann

Reviewed By: dhaumann

Subscribers: dhaumann, kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D17939