QueryAlias.get() should return nullable columns (Column<T?> or ExpressionAlias<T?>)

If you have aliased a query it could be part of a join which may result a column being null even though that column is not nullable in the original table.
Therefore when retrieving a column from an Aliased query that column should be of nullable type. The unchecked cast that is part of that implementation at the moment could lead to NullPointerExceptions.
Also when you do check for null after the get idea tells you it is a useless check.