The result type computation for `COALESCE()` is incorrect

[RobertBrunel]

The type computation for COALESCE() currently falls back to VariadicFunctionValue#getResultType():

@Override                                                                                                                                                           
public Type getResultType() {
  return children.get(0).getResultType();
}

This returns the type of the first argument. For COALESCE() that is not entirely correct because it will usually get the nullability wrong. The result type of COALESCE() is not-nullable when any argument is non-nullable. A common idiom is COALESCE(…, 0), whose result is not nullable.

Fixing this should unlock some opportunities for simplifying and constant-folding expressions containing COALESCE().