Suggestion for Handling Empty Data and JSON Decoding in `request_json`

1. Checking for empty data before attempting decoding, which can prevent unnecessary errors.
2. More explicit handling of the case when the data is empty.

For example:

local function request_json(req)
    local data = req:read_cached()
    if data == '' then
        return {}
    end

    local s, json = pcall(json.decode, data)
    if not s then
        error(sprintf("Can't decode json in request '%s': %s",
           data, tostring(json)))
       return nil
    end
    return json
end

This logic introduces optimization by avoiding unnecessary pcall invocation. It also allows handling a 500 error. However, of course, this requires attention from the developers of tarantool/http. I do not guarantee that the above code will not break some logic.

Start at the request_json entry point and inspect how req:read_cached() supplies data before json.decode is called. Compare empty-data and malformed-JSON behavior, then verify that the intended empty response and 500-error handling are covered without breaking existing request handling.