Remove redundant nil check for child_diff

[preciz]

child_diff is always a map.

[SteffenDE]

I don't think it is immediately obvious why it can't be nil, can you elaborate?

[preciz]

Here traverse_dynamic is called with 6 arguments:

phoenix_live_view/lib/phoenix_live_view/diff.ex

Lines 780 to 788 in 8d2ecf2

	{_counter, child_diff, child_prints, pending, components, template} =
	traverse_dynamic(
	render.(vars_changed, changed?),
	child_prints,
	pending,
	components,
	template,
	changed?
	)

Because of this diff is initialized to %{} here:

phoenix_live_view/lib/phoenix_live_view/diff.ex

Lines 640 to 642 in 8d2ecf2

	defp traverse_dynamic(dynamic, children, pending, components, template, changed?) do
	traverse_dynamic(dynamic, 0, %{}, children, pending, components, template, changed?)
	end

And then it is never set to nil if I follow the code correctly.