PDDL Parser: more explicit error when facing empty expressions

[victorpaleologue]

Supersedes https://github.com/aibasel/downward/pull/43

[victorpaleologue]

As discussed in http://issues.fast-downward.org/issue1030, we do the check only for goals, where the absence of a goal is clearly not supported (and perhaps should not). For other conditions, the spec is not clear so nothing was changed.