Fixed database schema

The answer fields in Questions are now NULL by default. The previous
defaults would not fullfill the forein key contraints on line 493 of
install.sql and thus the creation of new (yet unanswered) questions was
not possible.

db/install.sql

@@ -288,8 +288,8 @@ CREATE TABLE IF NOT EXISTS `Questions` (
   `QID` bigint(20) NOT NULL AUTO_INCREMENT,
   `UID` int(11) NOT NULL DEFAULT '0',
   `Question` text NOT NULL,
-  `AID` int(11) NOT NULL DEFAULT '0',
-  `Answer` text NOT NULL,
+  `AID` int(11) DEFAULT NULL,
+  `Answer` text,
   PRIMARY KEY (`QID`),
   KEY `UID` (`UID`),
   KEY `AID` (`AID`)