All categories page does not work when single category budget is set to $0

[ActuallyConnor]

When the budget is set to $0 for a category and then you try to go to the list of all the categories it errors out. If you go back to that individual category and set the value to a positive number it no longer errors out.

[pfaffenrodt]

I take a look

[Saibamen]

Please copy and paste stack trace here

[pfaffenrodt]

I fixed the calculation. Maybe it would be better, not even to allow to create or update a budget to a value below or equal of zero

[sbine]

Fixed in #43, thanks! I'm OK with allowing category budgets of 0 since you might want to skip budgeting for a month or two but not delete the category.

The budgeting feature in bdgt needs an overhaul anyway, I have a prototype implemented at /budget but don't yet have a good plan to integrate it into the rest of the UI.