[layout] Fix legend item properties' title entry not showing

the default "Legend" title upon item creation.

Author: nirvn

src/core/layout/qgslayoutitemlegend.cpp

@@ -42,6 +42,8 @@ QgsLayoutItemLegend::QgsLayoutItemLegend( QgsLayout *layout )
   connect( &layout->atlasComposition(), &QgsAtlasComposition::renderEnded, this, &QgsLayoutItemLegend::onAtlasEnded );
 #endif
 
+  mTitle = mSettings.title();
+
   // Connect to the main layertreeroot.
   // It serves in "auto update mode" as a medium between the main app legend and this one
   connect( mLayout->project()->layerTreeRoot(), &QgsLayerTreeNode::customPropertyChanged, this, &QgsLayoutItemLegend::nodeCustomPropertyChanged );