Query: Fixes ORDER BY undefined (and mixed type primitives) continuation token support

[bchong95]

Query: Fixes ORDER BY undefined continuation token support

Description

For ORDER BY queries we only store the continuation token for the partition we are resuming on. For the other partitions (left of target and right of target) we generate a filter that mimics the resume term. The problem here is that we didn't account for the undefined case when writing our visitor pattern. The following code results in a NullReferenceException:

orderByItem.Accept(cosmosElementToQueryLiteral);

Since orderByItem is null.

One solution to avoid the NRE is to use the undefined literal in the filter. Unfortunately this would lead to correctness issues, since comparing against undefined like so:

c.item > undefined

is also undefined. leading to the entire query returning no results.

So the solution is to use the IS_DEFINED system function to remove the undefined fields when needed.