You can't have empty result set name. I assume this is related to your previous question where you were getting
t=# select json_object_agg(code,value) from tt;
json_object_agg
----------------------------------------------------
{ "ALLOW_MAC_ADDR" : "1", "USER_ALIAS" : "James" }
So if you try to name the column with empty value (closest I can think of from your post requirements), you get the error:
t=# select json_object_agg(code,value) "" from tt;
ERROR: zero-length delimited identifier at or near """"
LINE 1: select json_object_agg(code,value) "" from tt;
^
Of course you can mokey hack it with space character, like:
t=# select json_object_agg(code,value) " " from tt;
----------------------------------------------------
{ "ALLOW_MAC_ADDR" : "1", "USER_ALIAS" : "James" }
which is ALMOST no name, but I assume you have to modify your "postgREST api" in order to do it properly - ignoring the result set attribute name