Settings: Handle empty Groups (#4576)

* export empty groups as an empty dict instead of crashing

* Update settings.py

Co-authored-by: black-sliver <59490463+black-sliver@users.noreply.github.com>

* check instance values from self as well

* Apply suggestions from code review

Co-authored-by: black-sliver <59490463+black-sliver@users.noreply.github.com>

---------

Co-authored-by: black-sliver <59490463+black-sliver@users.noreply.github.com>

Author: qwint

settings.py

@@ -109,7 +109,7 @@ def changed(self) -> bool:
     def get_type_hints(cls) -> Dict[str, Any]:
         """Returns resolved type hints for the class"""
         if cls._type_cache is None:
-            if not isinstance(next(iter(cls.__annotations__.values())), str):
+            if not cls.__annotations__ or not isinstance(next(iter(cls.__annotations__.values())), str):
                 # non-str: assume already resolved
                 cls._type_cache = cls.__annotations__
             else:
@@ -270,11 +270,15 @@ def dump(self, f: TextIO, level: int = 0) -> None:
             # fetch class to avoid going through getattr
             cls = self.__class__
             type_hints = cls.get_type_hints()
+            entries = [e for e in self]
+            if not entries:
+                # write empty dict for empty Group with no instance values
+                cls._dump_value({}, f, indent="  " * level)
             # validate group
             for name in cls.__annotations__.keys():
                 assert hasattr(cls, name), f"{cls}.{name} is missing a default value"
             # dump ordered members
-            for name in self:
+            for name in entries:
                 attr = cast(object, getattr(self, name))
                 attr_cls = type_hints[name] if name in type_hints else attr.__class__
                 attr_cls_origin = typing.get_origin(attr_cls)