IndexBuildInterceptor does not faithfully preserve multikey when a document generates no keys

IndexBuildInterceptor makes an incorrect assumption that a document must generate keys to be considered multikey.

In particular, sparse compound indexes may not generate keys, but will consider a document to be multikey[1]. MongoDB's validation code is strict and will compare an index's multikey to the multikey output of every document.

[1] Consider the index {a: 1, b: "2dsphere"} (2dsphere makes an index "auto-sparse"). Consider the document {_id: 1, a: [1,2]}. Because b is omitted, the sparse-ness will result in no index keys being generated. However, because a is an array, that field of the compound index will be considered to be multikey.