[FIX] fixed misleading warning ('Removed x peptide identifications...

... without sequence')

Cause of the problem: When parsing Mascot XML, blank PeptideIdentifications are created according to the number of search queries ("NumQueries" element in the XML). But potentially there are only results in the file for a fraction of those (e.g. due to filtering). The remaining blank IDs would be reported as "IDs without sequence" by the converter.

Fix: Make sure the blank IDs stay really empty, and don't count the empty ones for the "without sequence" number.