Change material RefreshIndictor.onRefresh signature from Future<Null> to Future<void> (#18775)

* Change material refresh indicator onRefresh signature from Future<Null> to Future<void>.

* Update authors.

* Explicitly name void argument.
diff --git a/AUTHORS b/AUTHORS
index ab63cf6..c8e76db 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -24,3 +24,4 @@
 Dan Field <dfield@gmail.com>
 Noah Groß <gross@ngsger.de>
 Victor Choueiri <victor@ctrlanddev.com>
+Lukasz Piliszczuk <lukasz@intheloup.io>