Add an optional builder for a custom "empty state" widget

[timovandeput]

What is the new or updated feature that you are suggesting?

Add an optional builder that allows for showing a custom widget when the list is empty.

Why should this feature be included?

In case of the "empty state" (=there are no items in the list), this would allow for a centered indicator that there currently is no data but the user could swipe down to check again.

Additional context

At the moment I am generating a list item for this state, but that cannot be centered on the screen.