fix(sys_mon): guard before calling `erlang:port_info`

Sometimes, the `emqx_sys_mon:procinfo/1` might be called with
something that is not a port, like `[]`.  Not sure on the conditions
for this to happen.

```
2022-02-18T20:05:02.671592+00:00 [error] Generic server emqx_sys_mon terminating. Reason: {badarg,[{erlang,port_info,[[]],[{error_info,#{module => erl_erts_errors}}]},{emqx_sys_mon,portinfo,1,[{file,"/emqx/apps/emqx/src/emqx_sys_mon.erl"},{line,205}]},{emqx_sys_mon,'-handle_info/2-fun-5-',2,[{file,"/emqx/apps/emqx/src/emqx_sys_mon.erl"},{line,150}]},{emqx_sys_mon,suppress,3,[{file,"/emqx/apps/emqx/src/emqx_sys_mon.erl"},{line,184}]},{gen_server,try_dispatch,4,[{file,"gen_server.erl"},{line,695}]},{gen_server,handle_msg,6,[{file,"gen_server.erl"},{line,771}]},{proc_lib,wake_up,3,[{file,"proc_lib.erl"},{line,236}]}]}. Last message: {monitor,<0.7796.0>,busy_dist_port,[]}. State: #{events => [{busy_dist_port,#Port<0.127>}],timer => #Ref<0.2758388682.1853620226.133920>}.
```

apps/emqx/src/emqx_sys_mon.erl

@@ -202,7 +202,12 @@ get_proc_lib_initial_call(Pid) ->
     end.
 
 portinfo(Port) ->
-    [{port, Port} | erlang:port_info(Port)].
+    PortInfo =
+        case is_port(Port) andalso erlang:port_info(Port) of
+            L when is_list(L) -> L;
+            _ -> []
+        end,
+    [{port, Port} | PortInfo].
 
 safe_publish(Event, WarnMsg) ->
     Topic = emqx_topic:systop(lists:concat(['sysmon/', Event])),

apps/emqx/test/emqx_sys_mon_SUITE.erl

@@ -35,6 +35,11 @@
                     {self(), busy_port,
                      fmt("busy_port warning: suspid = ~p, port = ~p",
                          [self(), ?FAKE_PORT]), ?FAKE_PORT},
+                    %% for the case when the port is missing, for some
+                    %% reason.
+                    {self(), busy_port,
+                     fmt("busy_port warning: suspid = ~p, port = ~p",
+                         [self(), []]), []},
                     {self(), busy_dist_port,
                      fmt("busy_dist_port warning: suspid = ~p, port = ~p",
                          [self(), ?FAKE_PORT]), ?FAKE_PORT},