Same as String.next_codepoint(str), next_grapheme(str) returns nil if str is empty.
https://github.com/elixir-lang/elixir/commit/87fee377114fd3c3e4f997d06799e25335dd2f8e
next_grapheme(str)
Same as next_codepoint, but returns graphemes (:no_graphemenil on completion)