Bibliospec/Skyline output fails when unmodified entries are empty instead of `-`

[RalfG]

Bibliospec/Skyline output fails when unmodified entries are empty instead of -

This works:

spec_id,modifications,peptide,charge
LSAGVEFLK,-,LSAGVEFLK,2

This does not:

spec_id,modifications,peptide,charge
LSAGVEFLK,,LSAGVEFLK,2

(tested on web server)