Send None for missing parent records instead of False

issue3448
review1761002
(transplanted from 207e9ee40735a5f9a37b796807f5f56189d25dba)

tryton/gui/window/view_form/model/record.py

@@ -496,7 +496,7 @@ class Record(SignalEvent):
             scope = values
             for i in arg.split('.'):
                 if i not in scope:
-                    scope = False
+                    scope = None
                     break
                 scope = scope[i]
             res[arg] = scope