Fix bug with exposure transitions - only perform if pop > 0

[claireh93]

No description provided.

[github-actions]

Pull Request Test Coverage Report for Build 4808223543

• 18 of 18 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 72.597%

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Totals
Change from base Build 4510763159:	0.02%
Covered Lines:	2591
Relevant Lines:	3569

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💛 - Coveralls

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (2b0e4aa) 80.61% compared to head (16dd594) 80.62%.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #113      +/-   ##
==========================================
+ Coverage   80.61%   80.62%   +0.01%     
==========================================
  Files          43       43              
  Lines        2971     2973       +2     
==========================================
+ Hits         2395     2397       +2     
  Misses        576      576              

Impacted Files	Coverage Δ
src/Transitions/EpiRun.jl	100.00% <100.00%> (ø)

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