[FIX] If balance_start is set by parser and equals 0.0, don't override it

In account_statement_base_import, there is a problem if a parser wants to set the balance_start to zero, the value won't be retained. I propose to fix that in this small patch.
`if 'balance_start' not in vals:` suffice and is a better idiom that using `keys()`, it will look by default in the keys.