consider default_style.name if set (#7394)

Co-authored-by: Giovanni Allegri <giohappy@gmail.com>
(cherry picked from commit 5c5f068)

geonode/maps/models.py

@@ -589,6 +589,8 @@ def get_legend(self):
             layer_obj = Layer.objects.filter(alternate=self.name).first()
             if ':' in style_name:
                 style_name = style_name.split(':')[1]
+            elif layer_obj.default_style:
+                style_name = layer_obj.default_style.name
             href = layer_obj.get_legend_url(style_name=style_name)
             return {style_name: href}
         except Exception as e: