fix truncate size nullable (#789)

Co-authored-by: Itamar Turner-Trauring <itamar@itamarst.org>
Co-authored-by: Hervé Beraud <hberaud@redhat.com>

eventlet/greenio/py3.py

@@ -158,6 +158,8 @@ def close(self):
             setattr(self, method, _operation_on_closed_file)
 
     def truncate(self, size=-1):
+        if size is None:
+            size = -1
         if size == -1:
             size = self.tell()
         try: