We should add an $isNull aggregation expression, that returns true for missing, null, or undefined values.

Original Description

I have documents that sometimes have an array field called purchases and sometimes not and it fails when $size hits a document without the purchases field. I don't see how to check if it is an array before running it. $exists does not seem to work within $project nor $type to check if it is an array type. The easiest would be if $size just doesn't fail and returns null.