Making "empty" titles disappear

[Br. Samuel Springuel]

I have a custom book titles whose definition looks like this:

    bookTitleMarkup = \markup { \fontsize #-3 \column {
        \fill-line { \fontsize #6 \fromproperty #'header:title }
        \fill-line {
            \fromproperty #'header:meter
            \concat { "Tune: " \fromproperty #'header:tune }
        }
        \fill-line {
            \concat { "Lyrics: " \fromproperty #'header:poet }
            \fromproperty #'header:composer
        }
        \fill-line {
            \override #'(line-width . 30) \wordwrap-field 
#'header:poet_copyright
            \override #'(line-width . 30) \wordwrap-field 
#'header:tune_copyright
        }
    }}

This is defined in a master style file for a project which is then 
loaded by each score in the project.  Ideally all those custom fields 
would be defined, but in some cases I've either not had the time or been 
able to fill them all out yet.  For most of the fields this is not a 
problem as the empty field takes up no space in the score.  The problem 
is for `tune` or `poet` header field which also have labels attached to 
them.  In those cases, I get the labels "Tune: " and "Lyrics: " still 
appearing in the score.  Ideally I'd like those labels to disappear when 
their associated field is empty.  Is there a way to make that happen?

%%% BEGIN snippet %%%
\version "2.19.53"

\paper  {
    bookTitleMarkup = \markup { \fontsize #-3 \column {
        \fill-line { \fontsize #6 \fromproperty #'header:title }
        \fill-line {
            \fromproperty #'header:meter
            \concat { "Tune: " \fromproperty #'header:tune }
        }
        \fill-line {
            \concat { "Lyrics: " \fromproperty #'header:poet }
            \fromproperty #'header:composer
        }
        \fill-line {
            \override #'(line-width . 30) \wordwrap-field 
#'header:poet_copyright
            \override #'(line-width . 30) \wordwrap-field 
#'header:tune_copyright
        }
    }}
}

\new Staff {
    c'
}

%%% END snippet %%%

--
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Br. Samuel, OSB
St. Anselm’s Abbey
Washington, DC
(R. Padraic Springuel)

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