[FIX] fix dereference of iterator

- vector iterator front() is not dereferencable if vector is empty
- thanks to @aiche for finding the error!
[FIX] fixed reading of empty spectra
[NOP] fixed indentation
Merge pull request #5 from aiche/hroest-fix/fixCachedMzML

[FIX] fixed reading of empty spectra
Merge pull request #842 from hroest/fix/fixCachedMzML

[FIX] fix dereference of iterator