Fix #153

[MKoesters]

• initialize chromatogram_count with None
• initialize spectrum_count with None
• supply tests for files with only chromatograms and files with spectra and chromtograms

[codecov[bot]]

Codecov Report

Merging #156 into master will increase coverage by 0.2%. The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #156     +/-   ##
=========================================
+ Coverage   82.95%   83.16%   +0.2%     
=========================================
  Files          32       32             
  Lines        3468     3475      +7     
=========================================
+ Hits         2877     2890     +13     
+ Misses        591      585      -6

Impacted Files	Coverage Δ
pymzml/run.py	86.5% <100%> (+3.78%)	:arrow_up:
tests/main_reader_test.py	99.32% <100%> (+0.02%)	:arrow_up:

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