Avoid server error in FlexibleImageWidget validation (#4381)

Fix error in `FlexibleImageWidget.decompress()` due to unexpected value.
This now properly raises a "field required" validation error on the
field.

The possible data returned from the possible widgets of
`FlexibleImageWidget` was not completely tested. This adds a test where
we extract the form data from the response of the flexible image widget
for each of the possible options the user can select.

Closes #4312

Author: koopmant

app/grandchallenge/cases/widgets.py

@@ -63,13 +63,17 @@ def __init__(
         )
         super().__init__(widgets, attrs)
 
-    def decompress(self, value):
+    def decompress(self, value):  # noqa: C901
         if not value:
             return [None, None]
 
         if isinstance(value, (list, tuple)):
             if len(value) == 1:
                 item = value[0]
+                if item == "":
+                    return [None, None]
+                if not item:
+                    raise RuntimeError("Unexpected value")
                 if item in ImageWidgetChoices.names:
                     return [None, None]
                 if Image.objects.filter(pk=item).exists():