`get_slot_changes` not initialized to null

[jacobmischka]

This results in a trailing comma in the function call, which is normally fine for modern envs but, ha, of course not in ancient ones.

Current output when the node has no values:

default_slot.p(get_slot_changes(default_slot_1, ctx, changed,), get_slot_context(default_slot_1, ctx, null));
                                                            ^