Fix detection of decorated function always returning None

Author: brianschubert

mypy/checkexpr.py

@@ -716,6 +716,8 @@ def always_returns_none(self, node: Expression) -> bool:
 
     def defn_returns_none(self, defn: SymbolNode | None) -> bool:
         """Check if `defn` can _only_ return None."""
+        if isinstance(defn, Decorator):
+            defn = defn.func
         if isinstance(defn, FuncDef):
             return isinstance(defn.type, CallableType) and isinstance(
                 get_proper_type(defn.type.ret_type), NoneType