bugfix: make MemorySubmissionResult.image_of_selected_name optional

For when the database does not contain an image for the selected name.

backend/backend/models.py

@@ -100,7 +100,7 @@ class MemorySubmissionResult(BaseModel):
     selected_name: MemoryName
     name_of_selected_image: MemoryName
     selected_image: str
-    image_of_selected_name: str
+    image_of_selected_name: str = None
 
 
 class TechniqueGameStatistic(BaseModel):