Deprecate bad function plugin arg

[Booplicate]

This is waiting for #9725, do not merge until then, it uses the new deprecation API.

Originally planned to remove this with r8, but I think it will be better if we give time and properly deprecate this parameter now.

Changes:

• in the following funcs, changed the _args/args parameter from [] to None (which then being set to an empty tuple). Mutable def arguments can cause bugs
• • functionplugin
• • registerFunction
• • setArgs
• setArgs is now deprecated
• getArgs is now deprecated

Testing:

• verify you get deprecation warning when using the parameter or the setArgs function

[ThePotatoGuy]

changed the _args parameter from [] to () (mutable def arguments can cause bugs)

for these, make it None in the params and do a None check to default - better than still leaving the door open for default arg weirdness.

The merge-base changed after approval.