Stopping a melismata extender

[Johan Vromans]

The following snippet:

  \score {
    <<
      \new Voice = HighVoice {
        \relative c' {
          c1 ~ | c1 | c1 | c1 |
        }
      }
      \lyricsto HighVoice \lyrics {
        aah __ _ _
      }
    >>
  }

This renders as:

   o     o     o    o
   aah_______________

I would like the extender to stop after the second note, as if the
third note did have some lyrics associated.

   o     o     o    o
   aah____

(Should you be interested why: from the 3rd note the lyrics are the
same as the previous voice, so there's no need to explicitly show
them).

I found one hack to accomplish this: give the third note a 'lyric'
syllable consisting of a single hard space. But I'm sure there are
better ways.

-- Johan