feat(rule_engine): rule sql add null function

apps/emqx_rule_engine/src/emqx_rule_funcs.erl

@@ -35,6 +35,7 @@
         , contains_topic/3
         , contains_topic_match/2
         , contains_topic_match/3
+        , null/0
         ]).
 
 %% Arithmetic Funcs
@@ -300,6 +301,9 @@ find_topic_filter(Filter, TopicFilters, Func) ->
         throw:Result -> Result
     end.
 
+null() ->
+    undefined.
+
 %%------------------------------------------------------------------------------
 %% Arithmetic Funcs
 %%------------------------------------------------------------------------------

apps/emqx_rule_engine/src/emqx_rule_utils.erl

@@ -228,6 +228,7 @@ tcp_connectivity(Host, Port, Timeout) ->
 unwrap(<<"${", Val/binary>>) ->
     binary:part(Val, {0, byte_size(Val)-1}).
 
+sql_data(undefined) -> null;
 sql_data(List) when is_list(List) -> List;
 sql_data(Bin) when is_binary(Bin) -> Bin;
 sql_data(Num) when is_number(Num) -> Num;