print bass staff not always

[Gianmaria Lari]

The following code prints three "lines" of music. The first and second lines are two staff system (treble and bass) and the third line just one staff (treble). 

\version "2.19.82"

rh = { \clef treble \repeat unfold 3 {a1 b1 c'1 d'1 \break}}

lh = { \clef bass s1*4 a1 1 11}

\score 

{ 

  \new PianoStaff <<

    \new Staff \rh

    \new Staff \lh

  >>

  \layout {}

}

I would like to avoid printing the first bass line because it's empty (s1*4). How can I do it?

Sorry for my "imprecise" english.

g.