implement may_be_none() for CloneNode

--HG--
extra : transplant_source : c%D4%7B%F2%21q%A8_%BD%3F%EE%E6K%EF%3Db%E6%AB%B4Y

Cython/Compiler/ExprNodes.py

@@ -7830,6 +7830,9 @@ class CloneNode(CoercionNode):
     def result(self):
         return self.arg.result()
 
+    def may_be_none(self):
+        return self.arg.may_be_none()
+
     def type_dependencies(self, env):
         return self.arg.type_dependencies(env)