luci-lib-jsonc: allow encoding empty lists

To be consistent with the behavior of luci-lib-json, an empty Lua table
should be encoded to an empty JSON list, not an empty JSON object.

To still allow encoding empty JSON objects, the usage of anything other
than a number or a string as a key (for example an empty table or a
function) can be used to force encoding as an object:

    json.stringify({})                  -- "[]"
    json.stringify({[{}] = true})       -- "{}"

Signed-off-by: Jan-Philipp Litza <janphilipp@litza.de>