Translate a.Any() with no predicate in filters.

For example:

collection.Where(x => x.A.Any())

translates to:

{ $match : {
    $and : [
        { A : { $ne : null } },
        { $nor : [ { A : { $size : 0 } } ] }
    ]
} }

 
The check for A not null is because we deem the null value to not match the Any condition.

The use of $nor with one argument is to simulate logical negation. We can't use $not because that has different semantics (and limitations) in the filter language.