fix: allow null values inside $expr objects

[MartinDrost]

Summary

Version 6.2.0 introduced a feature which casts literals inside $expr queries to their respective types based on the models schema. This pull request fixes an issue with this feature where JS would throw a null-reference error when using null as a value inside $ifNull arrays in the expression.

The implemented fix is non-intrusive an solely checks if the value which is to be cast is null and preserves the value if it is.

Examples

A common scenario is reproducing an $exists operator in an $expr using $ifNull. Let's say we want to fetch all books which don't have an authorId or where the authorId is null.

await bookModel.find({ $expr: { $eq: [ { $ifNull: [ "$authorId", null ] }, null ] } })

Executing this code will output the following error

TypeError: Cannot read properties of null (reading '$cond')
    at _castExpression (E:\project\node_modules\mongoose\lib\helpers\query\cast$expr.js:85:11)
    at E:\project\node_modules\mongoose\lib\helpers\query\cast$expr.js:94:26
    at Array.map (<anonymous>)
    at _castExpression (E:\project\node_modules\mongoose\lib\helpers\query\cast$expr.js:94:17)
    at castComparison (E:\project\node_modules\mongoose\lib\helpers\query\cast$expr.js:206:12)
    at _castExpression (E:\project\node_modules\mongoose\lib\helpers\query\cast$expr.js:105:18)
    at cast$expr (E:\project\node_modules\mongoose\lib\helpers\query\cast$expr.js:76:10)
    at cast (E:\project\node_modules\mongoose\lib\cast.js:87:13)
    at model.Query.Query.cast (E:\project\node_modules\mongoose\lib\query.js:5312:12)
    at model.Query.Query._castConditions (E:\project\node_modules\mongoose\lib\query.js:2198:10)

[vkarpov15]

Thanks 👍