Allow empty trajectories (closes #150)

[Enchufa2]

Requires some testing.

[codecov]

Codecov Report

Merging #151 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #151   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          52     52           
  Lines        2824   2828    +4     
=====================================
+ Hits         2824   2828    +4

Impacted Files	Coverage Δ
inst/include/simmer/activity.h	100% <100%> (ø)	⬆️
inst/include/simmer/activity/resource.h	100% <100%> (ø)	⬆️
inst/include/simmer/activity/branch.h	100% <100%> (ø)	⬆️
inst/include/simmer/process/datasrc.h	100% <100%> (ø)	⬆️
inst/include/simmer/activity/fork.h	100% <100%> (ø)	⬆️
inst/include/simmer/process/generator.h	100% <100%> (ø)	⬆️
inst/include/simmer/activity/async.h	100% <100%> (ø)	⬆️
R/utils.R	100% <100%> (ø)	⬆️
inst/include/simmer/process/arrival.h	100% <100%> (ø)	⬆️

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