converting example with empty nodes to forest 2.0

Question

I use the following code to produce this figure:

\documentclass{article}

\usepackage{forest}

\forestset{
sn edges/.style={for tree={parent anchor=south, child anchor=north,base=top},
             where n children=0{tier=word}{}
             }}

% auxiliary nodes without node label
\forestset{
  empty nodes/.style={
    delay={where content={}{shape=coordinate,for parent={for children={anchor=north}}}{}}}
}


\begin{document}

\begin{forest}
sn edges, empty nodes
[{} 
 [{} [{} [Mummy]]
     [{} [must]]
     [{} [leave]]]
 [{} [now]]]
\end{forest}

\end{document}

This works nicely for forest 1.0 (e.g. texlive 2013) but I cannot get it to work for forest 2.0.

This is what I tried:

\documentclass{article}

\usepackage{forest}

\useforestlibrary{linguistics}
\forestapplylibrarydefaults{linguistics}

\forestset{
sn edges/.style={for tree={parent anchor=south, child anchor=north,base=top},
             where n children=0{tier=word}{}
             }}


\begin{document}

\begin{forest}
sn edges
[{}, nice empty nodes 
 [{} [{} [Mummy]]
     [{} [must]]
     [{} [leave]]]
 [{} [now]]]
\end{forest}

\end{document}

Answer 1

Define empty nodes as

\forestset{
    empty nodes/.style={
    for tree={calign=fixed edge angles},
    delay={where content={}{shape=coordinate,for siblings={anchor=north}}{}}
    }
}

See this answer for details.