If the rpc in `emqx_dashboard_monitor_api:get_collect/1` fails, an
empty map is return. But the current function expects a 4-tuple,
which results in a 500 error returned on such occasions.
```
curl -su admin:public localhost:8888/api/v5/monitor/current | jq .
{
"code": "INTERNAL_ERROR",
"message": "error, function_clause, [{emqx_dashboard_monitor_api,format_current_metrics,[[#{},{0,0,0,0},{0,0,0,0}],{0,0,0,0}],[{file,\"emqx_dashboard_monitor_api.erl\"},{line,179}]},{emqx_dashboard_monitor_api,current_counters,2,[{file,\"emqx_dashboard_monitor_api.erl\"},{line,167}]},{minirest_handler,apply_callback,3,[{file,\"minirest_handler.erl\"},{line,112}]},{minirest_handler,init,2,[{file,\"minirest_handler.erl\"},{line,38}]},{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}]}]"
}
```
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