Avoid warnings being logged by convert_toggles_to_actions

[Clorith]

We have a filter that triggers Moderators::convert_toggles_to_actions(), which expects 3 arguments every time, but the filter doesn't always supply 3 arguments.

This patch makes the 3rd (and actually unused at this time) argument optional to avoid lots of warnings being logged (or output if you're working in a development environment).

The warning it throws, for reference:

Warning: Missing argument 3 for WordPressdotorg\Forums\Moderators::convert_toggles_to_actions()

[Otto42]

In 7482:

Support Forums: Correct arguments for function to avoid warnings being thrown. Fixes #3724. props Clorith