Make dicom_image_set optional (#4370)

When updating the models.py for gcapi I noticed that the dicom_image_set
is not optional:

```Python
class HyperlinkedImage(BaseModel):
    pk: str
    name: str
    files: list[ImageFile]
    dicom_image_set: DICOMImageSet
    width: Optional[int]
    height: Optional[int]
    depth: Optional[int]
    ...
```

Author: chrisvanrun

app/grandchallenge/cases/serializers.py

@@ -42,7 +42,7 @@ class Meta:
 class HyperlinkedImageSerializer(serializers.ModelSerializer):
     files = ImageFileSerializer(many=True, read_only=True)
     modality = ImagingModalitySerializer(allow_null=True, read_only=True)
-    dicom_image_set = DICOMImageSetSerializer(read_only=True)
+    dicom_image_set = DICOMImageSetSerializer(allow_null=True, read_only=True)
 
     class Meta:
         model = Image