Fixes issue #8065: Answer code conversion to '0'

DEV: when exporting to CSV/EXCEL etc. and conversion
DEV: of Y or N to '0' was chosen, values were wrongly
DEV: exported as 'missing values' (empty)
DEV:
DEV: nasty php typing problem....probably same problem
DEV: with issue #8039
small change to also honor 'false'

thanks to Shnoulle
use is_null instead of !isset
Merge pull request #100 from mfaber/zeroconversion

Fixed issue #8065: Answer code conversion to '0'