How much is the overhead of an Elastic search empty index?
In my personal experience with ES 1.x and 2.x the per shard overhead is not trivial and is usually in the range of a few MB/shard. As I understand it, this is memory reserved for indexing buffers, state metadata, references to lucene objects, caching objects, etc.
Basically a bit of memory is reserved to be able to index quickly and start caching if it is needed. I don't know how much of this is still true in the 5.x version.