Problem Statement/Rationale

type IndexDocument struct {
	Options                 bson.M `bson:",inline"`
	Key                     bson.D `bson:"key"`
	PartialFilterExpression bson.D `bson:"partialFilterExpression,omitempty"`
}

The definition of IndexDocument is incorrect that if an index document has `partialFilterExpression` set to an empty document, the field would be omitted when marshaling it to BSON. For TTL indexes in time-series collections, this results in mongorestore failure to restore such indexes.

Steps to Reproduce

See SERVER-102799

Expected Results

Mongorestore should restore indexes with `partialFilterExpression` option correctly

Actual Results

Mongorestore failure:

Failed: ttl_test_demo.sensor_data: error creating indexes for ttl_test_demo.sensor_data: createIndex error: (InvalidOptions) TTL indexes on time-series collections require a partialFilterExpression on the metaField