Handle null values when generating report string from task metrics

[turtlemonvh]

Fixes #20

[codecov-io]

Codecov Report

Merging #21 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #21   +/-   ##
=======================================
  Coverage   17.29%   17.29%           
=======================================
  Files           5        5           
  Lines         370      370           
  Branches       48       48           
=======================================
  Hits           64       64           
  Misses        306      306

Impacted Files	Coverage Δ
.../main/scala/ch/cern/sparkmeasure/taskmetrics.scala	0% <ø> (ø)	⬆️

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[LucaCanali]

Thanks for the PR.

[turtlemonvh]

Thanks for the merge @LucaCanali !