Empty lyrics lead to inaccurate 'cannot find Voice' warning

[Sietse Brouwer]

Dear bug auditor,

This bug concerns Lilypond 2.12.3. It may affect later versions, I
have been unable to test this.
When I add an empty set of lyrics to a voice with \addlyrics, the
compiler gives the following warning:
bug.ly:14:16: warning: cannot find Voice `vSoprano'
when in fact the problem is that the Lyrics expression is empty

Why I think this could be a problem: I was typesetting a song in
parts, and I'd made some dummy variables for the verses I had yet to
enter. This warning cost me half an hour as I tried to figure out what
was wrong with how I defined the Voice's name.

Desired behaviour: a warning like "Warning: skipping empty lyrics"
would be more appropriate, I think, especially as that is indeed what
seems to happen. (See example below. Verse C appears right under verse
A.)

Kind regards,

Sietse
Sietse Brouwer

<begin tiny example>
%Lilypond problem: inaccurate warning when lyrics are empty

verseOne = \lyricmode { This is verse A }
verseTwo = \lyricmode { } % empty verse
verseThree = \lyricmode { This is verse C }

\score {
  \new ChoirStaff <<
    \new Staff = "upper" <<
        \new Voice = "vSoprano" { a b c' d' }
    >>
    \new Lyrics \lyricsto "vSoprano" { \verseOne }
    \new Lyrics \lyricsto "vSoprano" { \verseTwo }
    \new Lyrics \lyricsto "vSoprano" { \verseThree }
  >>
}
<end tiny example>

<begin output transcript>
GNU LilyPond 2.12.3
Processing `bug.ly'
Parsing...
Interpreting music...
bug.ly:15:16: warning: cannot find Voice `vSoprano'

    \new Lyrics
                \lyricsto "vSoprano" { \verseTwo }
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
Layout output to `bug.ps'...
Converting to `./bug.pdf'...
<end output transcript>