Fix empty list handling in command map normalization

[haoxianhan]

Summary

Fix command map normalization so empty lists stay [] instead of becoming #{}.

Minimal Reproduction

Using mc_worker_api interface:

{ok, Conn} = mc_worker_api:connect([
  {database, <<"test">>},
  {host, "127.0.0.1"},
  {port, 27027},
  {use_legacy_protocol, false}
]).

Collection = <<"tuple_norm_fix_repro">>.

{{true, _}, _} = mc_worker_api:insert(Conn, Collection, #{
  <<"_id">> => 1,
  <<"test_0">> => 0,
  <<"test_1">> => <<"hello">>,
  <<"test_3">> => []
}).

{true, Res} = mc_worker_api:command(Conn, #{
  <<"findAndModify">> => Collection,
  <<"query">> => #{<<"test_0">> => 0},
  <<"update">> => #{
    <<"$set">> => #{
      <<"test_1">> => <<"hello111">>,
      <<"test_3">> => []
    }
  },
  <<"new">> => true
}).

Before this fix, maps:get(<<"value">>, Res) contained:

#{<<"_id">> => 1,
  <<"test_0">> => 0,
  <<"test_1">> => <<"hello111">>,
  <<"test_3">> => #{}}

After this fix, the same call returns:

#{<<"_id">> => 1,
  <<"test_0">> => 0,
  <<"test_1">> => <<"hello111">>,
  <<"test_3">> => []}

`findAndModify` is one concrete reproduction path; the root cause is in shared
command map normalization.

[comtihon]

thank you!