Issue #374: Add test of input mu ~ 1 as formula

[athowes]

Description

This PR closes #374.

I think we already had things set up to work fine with formula = mu ~ 1 because epidist_formula is calling:

formula <- brms:::validate_formula(formula, data = data, family = family)

So it already is going to convert from mu ~ 1 to brms::bf.

I added one unit test to check this.

Checklist

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