In a persistence test, I check whether a MongoDB index has been correctly initialized. For this purpose, I like to use the IndexInfo class and its equals-method. But this does not work currently, because the expected index created by me has a partialFilterExpression of null, but the actual one loaded from MongoDB has a partialFilterExpression of "". As a workaround, I now compare field by field, ignoring partialFilterExpression.
I would suggest to set partialFilterExpression to null (instead of "") when it is not defined by the MongoDB index.