Allow bills with an amount of zero

[zorun]

Bills with an amount of zero may be useful to remember that a transaction happened on a specific date, while the amount doesn't matter.

I use that with per-year projects when a reimbursement happens in year N but is relative to year N-1: I record it with an amount of zero in the project of year N, and with the real amount in the project of year N-1.

Besides, it's already possible to create such bills: while "0" is refused, "0.0" is accepted. There are no visible issues with this kind of bills.

[Glandos]

I've reviewed the history of this, and it's quite funny:

• b88b317 fixes User shouldn't be able to add negative bills #45 and add a test to < 0
• then it was reopened because some people needed negative bills, but Allow negative bill amounts #150 transformed this in == 0.
• In the end, the tests was broken by some dependency upgrade, and I push == "0" with a new test in Null amount validator was broken #990 without thinking about the necessity of this test.

Go for it. It might be silly to some, but harmless anyway!

[zorun]

Thanks for the historical perspective :D

I had to rebase, I will merge when the CI completes.