Improve Fuzzy parsing with invalid strings

[elfassy]

Why

The fuzzy parsing tries to guess the amount from a string. We can get a little more strict about the guesses we're making

What

• BREAKING CHANGE: In cases where it's not able to make a guess at all, we should return nil instead of 0

[TayKangSheng]

Okay. With this change, we will need to check all the places that calls Fuzzy.parse to check for nil value and handle with a fallback when necessary.

[elfassy]

I'm going to split the nil behaviour in it's own PR. I'd still like to get the other changes in v3

EDIT: pr -> #327