/schema endpoint should always return an array for indexes or fields

[yuce]

Description

This is probably low priority, but /schema endpoint returns the following when there is no index:

$ curl localhost:10101/schema
{"indexes":null}

But returns the following if there is any index:

$ curl localhost:10101/index/i1 -d ''
$ curl localhost:10101/schema
{"indexes":[{"name":"i1","fields":null}]}

It should return empty array [] for consistency if there are no indexes; the same is true for fields.

Success criteria (What criteria will consider this ticket closeable?)

• Return empty array if there are no indexes.
• Return empty array if an index has no fields.