feat: support None as return type, such as def func() -> None:

PiperOrigin-RevId: 764365009

Author: google-genai-bot

google/genai/_automatic_function_calling_util.py

@@ -37,6 +37,7 @@
     bool: types.Type.BOOLEAN,
     list: types.Type.ARRAY,
     dict: types.Type.OBJECT,
+    None: types.Type.NULL,
 }