Add nullptr default to all undo-manager arguments in ValueTree

[ImJimmi]

Adds default nullptr value for all UndoManager* arguments in public-facing ValueTree methods. Having to explicitly supply the nullptr whenever you interact with a value tree makes code very messy, and is a bit of a code smell, especially for devs who are less familiar with JUCE as it's not at all obvious what that argument is.

Discussed here: https://forum.juce.com/t/fr-default-undomanager-arguments-to-nullptr-in-all-valuetree-methods/51689