Fix edge case where error is thrown on empty choice field

Author: Archmonger

src/reactpy_django/forms/utils.py

@@ -14,8 +14,9 @@ def convert_multiple_choice_fields(data: dict[str, Any], initialized_form: Form
 
     # Convert multiple choice field text into a list of values
     for choice_field_name in multi_choice_fields:
-        if not isinstance(data.get(choice_field_name), list):
-            data[choice_field_name] = [data[choice_field_name]]
+        value = data.get(choice_field_name)
+        if value is not None and not isinstance(value, list):
+            data[choice_field_name] = [value]
 
 
 def convert_boolean_fields(data: dict[str, Any], initialized_form: Form | ModelForm) -> None: