lljson.empty_array_mt behaves like lljson.array_mt

[Suzanna-Linn]

local tab = { 1, k ="val" }
setmetatable(tab, lljson.empty_array_mt)
print(lljson.encode(tab))
--> [1]

I expected empty_array_mt to work only on empty tables but it always encodes an array, in practice doing the same than array_mt.

It seems that it's not checking for the length:

slua/VM/src/cjson/lua_cjson.cpp

Lines 1182 to 1201 in bdd984b

	len = lua_array_length(l, cfg, json);
	if (len > 0 || (len == 0 && !cfg->encode_empty_table_as_object)) {
	json_append_array(l, cfg, current_depth, json, len, raw);
	} else {
	if (has_metatable) {
	lua_getmetatable(l, -1);
	lua_pushlightuserdatatagged(l, json_lightudata_mask(JSON_EMPTY_ARRAY), LU_TAG_JSON_INTERNAL);
	lua_rawget(l, LUA_REGISTRYINDEX);
	as_array = lua_rawequal(l, -1, -2);
	lua_pop(l, 2); /* pop pointer + metatable */
	if (as_array) {
	len = lua_objlen(l, -1);
	raw = 1;
	json_append_array(l, cfg, current_depth, json, len, raw);
	break;
	}
	}
	json_append_object(l, cfg, current_depth, json);
	}

len seems to be -1 in this case, perhaps this would work?

if (len == 0 && has_metatable) {