Major Description
For property indexes, setting the config option "declaringNodeTypes" to an empty array will cause the index not to index any data, but will still allow the index to be used for all node type. That means, the index can be used, and would return no results. Queries would therefore return an empty result, when this index is used (even if there are nodes that match).
The problem is that the interpretation of an existing, but empty declaringNodeTypes property is different in the index editor and in the index cost / lookup operation.