Fix error with sql when checking should_update for ended shows (this never...

Fix error with sql when checking should_update for ended shows (this never worked before, everything was ALWAYS null because of checking is some values where = 'x' and the columns are numeric
Also, make it 0 when the WHERE causes it to have no rows to calculate min or max
Fixes https://github.com/SickRage/sickrage-issues/issues/151