fix: use MultiUserServerConfig for SS2022 with empty clients array

[Copilot]

When a Shadowsocks 2022 inbound is configured with "clients": [], buildShadowsocks2022() creates a single-user ServerConfig (no UserManager) instead of MultiUserServerConfig, making it impossible to add users via gRPC API.

Root cause: len(v.Users) == 0 is true for both a nil slice (field omitted) and an empty slice ("clients": []). Go's JSON unmarshaler distinguishes these — omitted yields nil, empty array yields [].

Changes:

• Changed guard from len(v.Users) == 0 to v.Users == nil so only a truly absent clients field produces single-user ServerConfig
• Added len(v.Users) == 0 || before v.Users[0].Address == nil to prevent index-out-of-range on empty slice and correctly route to MultiUserServerConfig

// Before: both nil and empty [] hit this branch
if len(v.Users) == 0 {
    config := new(shadowsocks_2022.ServerConfig) // no UserManager

// After: only nil (field omitted) hits single-user path
if v.Users == nil {
    config := new(shadowsocks_2022.ServerConfig)

// Empty [] now falls through to multi-user path
if len(v.Users) == 0 || v.Users[0].Address == nil {
    config := new(shadowsocks_2022.MultiUserServerConfig) // implements UserManager

Added TestShadowsocks2022ServerConfigParsing covering both cases.

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