Fix the init_proc_mng_policy bug

Prior to this change, when the plugin like emqx_sn_gateway which has
no zone run the init_proc_mng_policy function, it would trigger error
and application crash.

This change add a case to avoid crash.

src/emqx_misc.erl

@@ -63,8 +63,14 @@ proc_stats(Pid) ->
 -define(DISABLED, 0).
 
 init_proc_mng_policy(Zone) ->
-    #{max_heap_size := MaxHeapSizeInBytes} = ShutdownPolicy =
-        emqx_zone:get_env(Zone, force_shutdown_policy),
+    #{max_heap_size := MaxHeapSizeInBytes}
+        = ShutdownPolicy
+        = case Zone of
+              undefined ->
+                  #{max_heap_size => 0};
+              _ ->
+                  emqx_zone:get_env(Zone, force_shutdown_policy)
+          end,
     MaxHeapSize = MaxHeapSizeInBytes div erlang:system_info(wordsize),
     _ = erlang:process_flag(max_heap_size, MaxHeapSize), % zero is discarded
     erlang:put(force_shutdown_policy, ShutdownPolicy),
@@ -106,4 +112,3 @@ is_enabled(Max) -> is_integer(Max) andalso Max > ?DISABLED.
 proc_info(Key) ->
     {Key, Value} = erlang:process_info(self(), Key),
     Value.
-