[FIX] PeptideIndexer crashes on empty idXML (#653)

a call to 

PeptideIndexer.exe -in c:\Temp\empty.idXML -fasta c:\temp\target_decoy_Ecoli_K12_TaxID_83333.proteomes.fasta -out c:\temp\pi.idXML

will now result in 
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Error: No peptides were matched to the decoy portion of the database! Did you provide the correct concatenated database? Are your 'decoy_string' (=_rev) and 'prefix' (=0) settings correct?
Set 'missing_decoy_action' to 'warn' if you are sure this is ok!
Quitting...
PeptideIndexer took 0.23 s (wall), 0.20 s (CPU), 0.03 s (system), 0.17 s (user).
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This is desired behaviour since the user should be made aware of an empty idXML.

Adding  "-missing_decoy_action warn" will have it finish successfully.