Fix and comment type mismatches in email library hints (#10437)

stdlib/email/charset.pyi

@@ -1,4 +1,6 @@
-from collections.abc import Iterator
+from collections.abc import Callable, Iterator
+from email.message import Message
+from typing import overload
 
 __all__ = ["Charset", "add_alias", "add_charset", "add_codec"]
 
@@ -14,10 +16,13 @@ class Charset:
     input_codec: str | None
     output_codec: str | None
     def __init__(self, input_charset: str = "us-ascii") -> None: ...
-    def get_body_encoding(self) -> str: ...
+    def get_body_encoding(self) -> str | Callable[[Message], None]: ...
     def get_output_charset(self) -> str | None: ...
     def header_encode(self, string: str) -> str: ...
     def header_encode_lines(self, string: str, maxlengths: Iterator[int]) -> list[str]: ...
+    @overload
+    def body_encode(self, string: None) -> None: ...
+    @overload
     def body_encode(self, string: str) -> str: ...
     def __eq__(self, other: object) -> bool: ...
     def __ne__(self, __value: object) -> bool: ...