When there are no budgets we were seeing an exception in the budgets’
index
There are two parts to take into account here:
1) Making sure there is a current_budget present, otherwise we display
the “no budgets” message
2) The map helper is called from the controller, so we need to make
sure current_budget is present there too
Note: We could have added a bunch of `try` statements in the budgets’s
index, instead of using a conditional, however there are quite a few
`current_budget` calls so it seems more appropriate to use a conditional
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