Aggregation operator for "is not null"

[Francesca_Ricci-Tam (Francesca Kukralova)]

Thanks @NeNaD and @Jason_Tran – indeed, using a $match right after the $search stage is exactly what I ended up doing in the end. : )
I realized that I was approaching it the wrong way – I shouldn’t be trying to filter on non-text parameters inside the $search stage (which would be primarily for text-based searching).