[Icon Item] Don't needlessly unset imagePath

Authored by broulik on Aug 30 2017, 2:39 PM.

Description

[Icon Item] Don't needlessly unset imagePath

We'll eventually delete the Plasma::Svg anyway if we failed to find an appropriate icon.
Just need to make sure we don't use isValid with the old source set but this is only done with
m_usesPlasmaTheme, hence the new check there, also optimizes the non-themed case.

Differential Revision: https://phabricator.kde.org/D7584