fix: /listeners API crash when there are nodes still in initial conf

[zhongwencool]

Fix GET /listeners API crashes when some nodes are still in init configuration.

{"code":"INTERNAL_ERROR","message":"error, {badmatch,{error,{'EXIT',{{badkey,<<\"listeners\">>},
[{erlang,map_get,[<<\"listeners\">>,#{}],[{error_info,#{module => erl_erts_errors}}]},{emqx_config,get_schema_mod,1,
[{file,\"emqx_config.erl\"},{line,505}]},{emqx_listeners,do_list_raw,0,[{file,\"emqx_listeners.erl\"},{line,92}]},
{emqx_listeners,list_raw,0,[{file,\"emqx_listeners.erl\"},{line,71}]},{emqx_mgmt_api_listeners,do_list_listeners,0,
[{file,\"emqx_mgmt_api_listeners.erl\"},{line,519}]}]}}}}, [{emqx_mgmt_api_listeners,listener_status_by_id,2,
[{file,\"emqx_mgmt_api_listeners.erl\"},{line,496}]},{emqx_mgmt_api_listeners,listener_status_by_id,1,
[{file,\"emqx_mgmt_api_listeners.erl\"},{line,472}]},{emqx_mgmt_api_listeners,list_listeners,2,
[{file,\"emqx_mgmt_api_listeners.erl\"},{line,342}]},{minirest_handler,apply_callback,3,[{file,\"minirest_handler.erl\"},
{line,111}]},{minirest_handler,handle,2,[{file,\"minirest_handler.erl\"},{line,44}]},{minirest_handler,init,2,
[{file,\"minirest_handler.erl\"},{line,27}]},{cowboy_handler,execute,2,[{file,\"cowboy_handler.erl\"},{line,41}]},
{cowboy_stream_h,execute,3,[{file,\"cowboy_stream_h.erl\"},{line,318}]},{cowboy_stream_h,request_process,3,
[{file,\"cowboy_stream_h.erl\"},{line,302}]},{proc_lib,init_p_do_apply,3,[{file,\"proc_lib.erl\"},{line,226}]}]"}