added conditional statement to avoid division by zero

[vgro]

Some experiments returned this warning, which is hopefully fixed here:

RuntimeWarning: invalid value encountered in scalar divide
  daily_rainfall *= rainfall / np.sum(daily_rainfall)

Fixes #869

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.79%. Comparing base (b490fda) to head (3fb2ff1).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #870   +/-   ##
========================================
  Coverage    94.79%   94.79%           
========================================
  Files           75       75           
  Lines         5589     5591    +2     
========================================
+ Hits          5298     5300    +2     
  Misses         291      291           

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[jacobcook1995]

LGTM!