[wtforms] also allow None for SelectField.choices

As suggested by srittau

Co-authored-by: Sebastian Rittau <sebastian.rittau@zfutura.de>

Author: marksteward

stubs/WTForms/wtforms/fields/choices.pyi

@@ -46,7 +46,7 @@ class SelectFieldBase(Field):
 
 class SelectField(SelectFieldBase):
     coerce: Callable[[Any], Any]
-    choices: Sequence[_Choice] | _GroupedChoices
+    choices: Sequence[_Choice] | _GroupedChoices | None
     validate_choice: bool
     def __init__(
         self,