make QLR files containing non-vector layers work again

This was a regression introduced in ff3200f.
(fixes #14091)

Author: SebDieBln

src/core/qgslayerdefinition.cpp

@@ -131,7 +131,7 @@ bool QgsLayerDefinition::loadLayerDefinition( QDomDocument doc, QgsLayerTreeGrou
   // Now that all layers are loaded, refresh the vectorjoins to get the joined fields
   Q_FOREACH ( QgsMapLayer* layer, layers )
   {
-    QgsVectorLayer* vlayer = static_cast< QgsVectorLayer * >( layer );
+    QgsVectorLayer* vlayer = dynamic_cast< QgsVectorLayer * >( layer );
     if ( vlayer )
     {
       vlayer->createJoinCaches();

tests/src/core/testqgsvectorlayerjoinbuffer.cpp

@@ -384,7 +384,7 @@ void TestVectorLayerJoinBuffer::testJoinLayerDefinitionFile()
   QList<QgsMapLayer*> mapLayers = QgsMapLayerRegistry::instance()->mapLayersByName( "layerB" );
   QCOMPARE( mapLayers.count(), 1 );
 
-  QgsVectorLayer* vLayer = static_cast<QgsVectorLayer*>( mapLayers.value( 0 ) );
+  QgsVectorLayer* vLayer = dynamic_cast<QgsVectorLayer*>( mapLayers.value( 0 ) );
   QVERIFY( vLayer );
 
   // Check for vector join