fix: Void functions return null instead of empty body (#1795)

* Change empty body reponse to null for scalar functions returning void or null values

* Add test for functions returning an integer with null value

Co-authored-by: Laurence Isla <lau.isla.c@gmail.com>

src/PostgREST/Private/QueryFragment.hs

@@ -98,7 +98,7 @@ asJsonF returnsScalar
 
 asJsonSingleF :: Bool -> SqlFragment --TODO! unsafe when the query actually returns multiple rows, used only on inserting and returning single element
 asJsonSingleF returnsScalar
-  | returnsScalar = "coalesce(string_agg(to_json(_postgrest_t.pgrst_scalar)::text, ','), '')::character varying"
+  | returnsScalar = "coalesce(string_agg(to_json(_postgrest_t.pgrst_scalar)::text, ','), 'null')::character varying"
   | otherwise     = "coalesce(string_agg(to_json(_postgrest_t)::text, ','), '')::character varying"
 
 asBinaryF :: FieldName -> SqlFragment

test/Feature/RpcSpec.hs

@@ -282,11 +282,18 @@ spec actualPgVersion =
           `shouldRespondWith`
             [json|{"id": 2}|]
 
-      it "returns nothing for void" $
+      it "returns null for void" $
         post "/rpc/ret_void"
             [json|{}|]
           `shouldRespondWith`
-            ""
+            "null"
+            { matchHeaders = [matchContentTypeJson] }
+
+      it "returns null for an integer with null value" $
+        post "/rpc/ret_null"
+            [json|{}|]
+          `shouldRespondWith`
+            "null"
             { matchHeaders = [matchContentTypeJson] }
 
       context "different types when overloaded" $ do
@@ -919,7 +926,7 @@ spec actualPgVersion =
         it "can set the same http header twice" $
           get "/rpc/set_cookie_twice"
             `shouldRespondWith`
-              ""
+              "null"
               { matchHeaders = [ matchContentTypeJson
                                , "Set-Cookie" <:> "sessionid=38afes7a8; HttpOnly; Path=/"
                                , "Set-Cookie" <:> "id=a3fWa; Expires=Wed, 21 Oct 2015 07:28:00 GMT; Secure; HttpOnly" ]}

test/fixtures/schema.sql

@@ -1051,6 +1051,10 @@ $$ language sql;
 
 create function test.ret_void() returns void as '' language sql;
 
+create or replace function test.ret_null() returns int as $$
+  select null::int;
+$$ language sql;
+
 create function test.ret_base64_bin() returns text as $$
   select i.img from test.images_base64 i where i.name = 'A.png';
 $$ language sql;