Fix `UserAttributeKey::getByHandle()` Return

Fix `UserAttributeKey::getByHandle()` return value to return false if
attribute key is not found. This maintains a consistent interface return
value as prior to change the first call before caching will returh -1
while following requests will return false

See
http://www.concrete5.org/developers/bugs/5-6-1
/userattributekeygetbyhandle-returns-1/