(nordigen) fix detection of -0.00 "debited" transactions (#744)

Check if the transaction amount is a positive 0 rather than -0.
I went for casting the string with `Number` but I could instead replace
the whole condition with `!trans.amount.startsWith('-')` or with
`trans.amount > 0 || trans.amount == '0.00'` if either of these variants
are preferred.

Fix for
https://github.com/actualbudget/actual/issues/724#issuecomment-1464907064