Use immutable default values for args

Default mutable values (e.g. arg1=[], arg2={}) could have side effects
in Python. So using None as a default value is safer.

Added hacking checks for default mutable args.

Closes: bug #1327473

Change-Id: I7055e534b91df794550de6c3b243324e582d4430
Merge "Use immutable default values for args"