fix(variform): undefined to empty string

apps/emqx_utils/src/emqx_variform.erl

@@ -96,6 +96,7 @@ eval_render(Expr, Bindings, Opts) ->
     end.
 
 %% Force the expression to return binary string (in most cases).
+return_str(X) when ?IS_EMPTY(X) -> <<"">>;
 return_str(Str) when is_binary(Str) -> Str;
 return_str(Num) when is_integer(Num) -> integer_to_binary(Num);
 return_str(Num) when is_float(Num) -> float_to_binary(Num, [{decimals, 10}, compact]);

apps/emqx_utils/test/emqx_variform_tests.erl

@@ -26,6 +26,18 @@
 render_test_() ->
     [
         {"direct var reference", fun() -> ?assertEqual({ok, <<"1">>}, render("a", #{a => 1})) end},
+        {"direct var reference missing", fun() ->
+            ?assertMatch({error, #{reason := var_unbound}}, render("a", #{}))
+        end},
+        {"direct var reference undefined", fun() ->
+            ?assertEqual({ok, <<"">>}, render("a", #{a => undefined}))
+        end},
+        {"direct var reference null", fun() ->
+            ?assertEqual({ok, <<"">>}, render("a", #{a => null}))
+        end},
+        {"direct var reference emptry str", fun() ->
+            ?assertEqual({ok, <<"">>}, render("a", #{a => <<>>}))
+        end},
         {"concat strings", fun() ->
             ?assertEqual({ok, <<"a,b">>}, render("concat(['a',',','b'])", #{}))
         end},