qmljs: Make string code more robust by checking isEmpty over isNull

Summary:
In all cases empty non-null QStrings match the tested cases as well, so the
code better catches those QString variants as well, not just empty null
ones.
Makes the code looking less fragile (no special handling of null QStrings)

Test Plan: Tests work/fail as before ;)

Reviewers: KDevelop, apol

Reviewed By: KDevelop, apol

Subscribers: kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D14277