Avoid adding empty parent icon theme

"Icon Theme/Inherits" may assign an empty value, e.g. Flattr icon
theme

Change-Id: I90713b3eb67e39d6a49a49d1f5b3d7a75a8f30f4
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/gui/image/qiconloader.cpp

@@ -223,6 +223,7 @@ QIconTheme::QIconTheme(const QString &themeName)
         // Parent themes provide fallbacks for missing icons
         m_parents = indexReader.value(
                 QLatin1String("Icon Theme/Inherits")).toStringList();
+        m_parents.removeAll(QString());
 
         // Ensure a default platform fallback for all themes
         if (m_parents.isEmpty()) {