Correctness of indexing zero-length string/binary keys

[katyo]

An attempt to index field values of zero-length causes LMDB error.
It because LMDB not support for that key length.

So currently zero-length values doesn't indexing, i.e. it is treated as nulls.
But that solution may have problems with some filters (like "$has") which differentiates nulls and empty values.

One of possible solutions is adding some character to all values (of string and binary types) when converting to keys.

[mbalex99]

Why don't you use a character to denote something like that? $0 That $ with a leading character seems reserved.

[katyo]

I think the trade-off solution is simply replace empty values by "\0" (or [0u8]). This is relatively harmless for duplicated keys and ordering. Also it harmless for strings because zero character too rarely presents in beginning of non-empty strings. Probably it may have some problems with unique binary keys because the [] and [0u8] is same in this case.