Validate timeUnit in ramping-arrival-rate

[mstoykov]

What?

Validate timeUnit in ramping-arrival-rate isn't 0 apart from not being negative.

Why?

Previous to this negative time units will error but 0 timeunit will panic.

This was fixed in constant-arrival-rate long ago, but nobody checked ramping-arrival-rate.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.59%. Comparing base (fc60f4d) to head (c4077e6).

❗ Current head c4077e6 differs from pull request most recent head e59a652. Consider uploading reports for the commit e59a652 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3658      +/-   ##
==========================================
+ Coverage   73.57%   73.59%   +0.01%     
==========================================
  Files         277      277              
  Lines       20247    20247              
==========================================
+ Hits        14897    14901       +4     
+ Misses       4402     4398       -4     
  Partials      948      948              

Flag	Coverage Δ
macos	73.49% <100.00%> (?)
ubuntu	73.51% <100.00%> (+<0.01%)	⬆️
windows	73.44% <100.00%> (+0.01%)	⬆️

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