Nullability

[divarvel]

Support for array and map types #135 implies the ability to query them. Since querying an array or a map is a partial operation, we need a way to handle errors / missing values.

Since biscuit datalog is untyped (for good reason imo, as much as i love types, i'm not sure how they could fit in the picture), a good solution for that would be to use a special null value for that.

Along with null, we need to define how it would interact with the rest of datalog:

• behaviour with == (imo: behave as other values and return true when compared with null, false otherwise)
• behaviour with other operators / methods (imo: behave as other values and blow up)
• should it be possible to define it as a literal, or should it be only introduced by faillible operations?
• defaulting operator (eg ??)

[Geal]

noting that it is related to #130 because if we test equality of null with any other types, then equality between different types should not return an invalid type error