Fix the unsafe nullability and unique analysis for ORDER BY expressions

[beikov]

Keyset pagination and optimizations regarding null precedence emulation depend on a working nullability and unique analysis of expressions.

Currently the analysis assumes that the owner of a path expression is non-optional i.e. rows are always produced for an alias. This is wrong and results in not being able to report unsafe use of ORDER BY items for keyset pagination. The nullability analysis might be wrong as it could report expressions being non-nullable because of the non-optional owner assumption. This leads to WHERE predicates for the keyset pagination that don't correctly handle nulls.

When the nullability analysis is fixed, we can also avoid generating null precedence handling for ORDER BY items.

Detecting nullability should be pretty simple. A path is nullable if the attribute is optional or one of the attributes parents in the join graph is optional. For *ToOne relations, a join node is optional if the FK is nullable. In general, relations are considered optional if they are left joined with a condition. Inner joined relations are always considered non-optional. A query root is always considered being an inner joined relation. Cross joined relations are also always non-optional.

A path is unique if the attribute is unique and the parents are all relations where a unique constraints can be inferred from the join predicate. Inner joins on one-to-one relations are always considered unique. Left joined relations are always considered non-unique as they could produce null values. Cross joined relations are always considered non-unique.

[beikov]

Note that it should also be possible to infer uniqueness when only *ToOne relations are joined from a root and the projections are based off that joined relation. This is the case when using e.g. an entity view with a custom view root.