Modify Foreign Keys to support null output.

[Lewis Dyer]

Foreign keys type is used to establish relationships between tables, When a foreign key is declared a cache is added to the source column (parent table).

Foreign keys in join tables are often replaced with range types with a window function but keeping the relationship allows the generator to correctly order the tables during generation.

Currently adding a foreign key would add a cache and output a value, need a way to turn off this behaviour.