fix: empty row on handler function

Closes https://github.com/PostgREST/postgrest/issues/3126

test/spec/Feature/Query/CustomMediaSpec.hs

@@ -119,6 +119,13 @@ spec = describe "custom media types" $ do
         , matchHeaders = ["Content-Type" <:> "application/json; charset=utf-8"]
         }
 
+    it "should not fail when the function doesn't return a row" $ do
+      request methodGet "/rpc/get_line?id=777" (acceptHdrs "application/vnd.twkb") ""
+        `shouldRespondWith` ""
+        { matchStatus = 200
+        , matchHeaders = ["Content-Type" <:> "application/vnd.twkb"]
+        }
+
   context "Proc that returns scalar based on a table" $ do
     it "can get an image with Accept: image/png" $ do
       r <- request methodGet "/rpc/ret_image" (acceptHdrs "image/png") ""

test/spec/fixtures/schema.sql

@@ -3517,6 +3517,11 @@ returns setof test.lines as $$
   select * from lines;
 $$ language sql;
 
+create or replace function test.get_line (id int)
+returns "application/vnd.twkb" as $$
+  select extensions.st_astwkb(geom)::"application/vnd.twkb" from lines where id = get_line.id;
+$$ language sql;
+
 create or replace function test.get_shop_bles ()
 returns setof test.shop_bles as $$
   select * from shop_bles;