Don't crash when working with non-KTp accounts

Summary:
Telepathy-Qt was not very happy when accountPath was an empty string: it crashed somewhere in Telepathy-Qt DBus handling code.
This simple check should avoid it.

Test Plan:
The crash was catched during work on T9595: [KAddressbook] Use KPeople model for contact list, see scratch repo and "testab" application there.
We weren't able to show info for non-KTp account. With this patch, it no longer crashes.

Reviewers: dvratil, davidedmundson

Reviewed By: davidedmundson

Subscribers: davidedmundson

Differential Revision: https://phabricator.kde.org/D15816