Stream pagination cursors don't support `undefined`

[ldanilek]

Cursors created by stream().query().paginate() call convexToJson on the index keys.
Index keys are typed as Value[] but actually they're (Value | undefined)[] -- it can be undefined if an indexed field is missing from some document. Handle this in types and in the cursor serde.

[ianmacartney]

Workarounds I've found:

• make it nullable instead of optional: add nulls explicitly
• use the paginator and track multiple cursors
• add an index that skips that field
• change the index order so you can use a normal paginated query (when e.g. you have [some_boolean, userId] and you want to paginate by userId and are using streams to do [true, userId ] and [false, userId]) - this may seem obvious but sometimes you don't realize you didn't need the old sorting