Fix one source of log spam referenced in Bug 388389 (Empty filename passed to function)

Summary:
I found two places in plasma-framework svg.cpp (Plasma::Svg) that were triggering this message.

path variable was not being checked for empty before passing to QFile::exists().

This seems to be one source of the issue reported in Bug 388389

Test Plan: Found that kicker was triggering this message and also plasmashell tooltips.

Reviewers: Frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: ngraham, alexeymin, aacid, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9928