Don't create multiple in-process backend instances

Summary:
The 'name' argument is in most cases empty by which the caller indicates that
they want the default backend. As such repeated calls to loadBackendInProcess(QString())
cause to instantiate a new backend every time, because the name check in the
condition is never true.

Reviewers: sebas, davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D9905