Allow min_wait and max_wait times of 0

[Aresius423]

I know this is a deprecated feature, but here's a fix anyways.

[codecov]

Codecov Report

Merging #1199 into master will decrease coverage by 0.4%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1199      +/-   ##
=========================================
- Coverage      79%   78.6%   -0.41%     
=========================================
  Files          20      20              
  Lines        1958    1958              
  Branches      308     308              
=========================================
- Hits         1547    1539       -8     
- Misses        335     339       +4     
- Partials       76      80       +4

Impacted Files	Coverage Δ
locust/core.py	87.71% <100%> (ø)	⬆️
locust/util/deprecation.py	85% <100%> (ø)	⬆️
locust/stats.py	84.35% <0%> (-1.82%)	⬇️

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[cyberw]

Thanks for your contribution!