Fixed issue #8039: Problems with subquestion code '0'

DEV: Both described issues were separate incidences of
DEV: the same thing: php empty()/($var) treat the '0'
DEV: as not set or empty.
DEV: isset() on the other hand also returns "" as true.
DEV: I am afraid this problem may be present more often in
DEV: the code. Maybe implement a custom function
DEV: is_set() which also returns FALSE for "" but TRUE for '0'
Merge pull request #102 from mfaber/SQ0meanstrouble205

Fixed issue #8039: Problems with subquestion code '0'