Shutup more annoying Qt warnings

(cherry-picked from cec59c0)

Author: nyalldawson

src/core/symbology/qgssymbollayerutils.cpp

@@ -3655,6 +3655,9 @@ QStringList QgsSymbolLayerUtils::listSvgFilesAt( const QString &directory )
 
 QString QgsSymbolLayerUtils::svgSymbolNameToPath( QString name, const QgsPathResolver &pathResolver )
 {
+  if ( name.isEmpty() )
+    return QString();
+
   // we might have a full path...
   if ( QFileInfo::exists( name ) )
     return QFileInfo( name ).canonicalFilePath();

tests/src/core/testqgsproject.cpp

@@ -211,6 +211,8 @@ void TestQgsProject::testPathResolverSvg()
   QString dataDir( TEST_DATA_DIR ); //defined in CmakeLists.txt
   QString layerPath = dataDir + "/points.shp";
 
+  QVERIFY( QgsSymbolLayerUtils::svgSymbolNameToPath( QString(), QgsPathResolver() ).isEmpty() );
+
   // build a project with 3 layers, each having a simple renderer with SVG marker
   // - existing SVG file in project dir
   // - existing SVG file in QGIS dir