Scheme function

[Víctor]

Hi all:

solfa =
#(define-music-function
    (parser location framed number note)
    (string? string? string?)
  #{
    ^\markup{
      \box
      \pad-to-box #'(0 . 0) #'(0 . 2.2){ #framed }
      #number :
      \italic #note
    }
  #})

\relative c' { c4^\solfa "C" "I" "do" }

I need this function to stop drawing the box when first argument is "" 
or stop printing the ":" when second or third argument are "". I'm very 
new to Scheme and any help is very much apreciated.

Regards,
Víctor.