fix error 500 with no image

Author: Lucie Laporte

sites_conformes/core/blocks/heros.py

@@ -207,7 +207,7 @@ def clean(self, value):
             raise StructBlockValidationError(block_errors=errors)
 
         if selected_background_type == "color":
-            value["image"] = None
+            value["image"] = {}
         elif selected_background_type == "image":
             value["background_color"] = None