KPluginMetaData: convert empty string to empty stringlist.

Summary:
As a fallback when the property type is unknown, we use string.
When reading a stringlist, if the key is absent or empty, this method
was returning QStringList(""), which was rather unexpected.

Test Plan:
XDG_DATA_DIRS='foo' ../bin/kplugininfotest in kservice:
Got dependencies ("")
FAIL! : KPluginInfoTest::testLoadDesktop(from KService::Ptr) Compared lists have different sizes.

Actual   (info.dependencies()) size: 1
Expected (QStringList()) size: 0

Reviewers: apol, mpyne, davidedmundson, arichardson

Reviewed By: arichardson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14722