fix void handling

[jryannel]

An empty type should be a void type. A void type should return void if the lang supports it or "" (empty string) otherwise.

• also fixes qt filters to use qreal instead of double

[jryannel]

In general a valid void type the absence of a typed symbol (e.g. no return) or and empty type (type: '') or any type with void (type: 'void').