Allow empty string object keys in json_object().

This makes the behaviour consistent with the json parser, other
json-generating functions, and the JSON standards.

src/backend/utils/adt/json.c

@@ -2184,10 +2184,6 @@ json_object(PG_FUNCTION_ARGS)
 					 errmsg("null value not allowed for object key")));
 
 		v = TextDatumGetCString(in_datums[i * 2]);
-		if (v[0] == '\0')
-			ereport(ERROR,
-					(errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
-					 errmsg("empty value not allowed for object key")));
 		if (i > 0)
 			appendStringInfoString(&result, ", ");
 		escape_json(&result, v);
@@ -2272,10 +2268,6 @@ json_object_two_arg(PG_FUNCTION_ARGS)
 					 errmsg("null value not allowed for object key")));
 
 		v = TextDatumGetCString(key_datums[i]);
-		if (v[0] == '\0')
-			ereport(ERROR,
-					(errcode(ERRCODE_NULL_VALUE_NOT_ALLOWED),
-					 errmsg("empty value not allowed for object key")));
 		if (i > 0)
 			appendStringInfoString(&result, ", ");
 		escape_json(&result, v);

src/test/regress/expected/json.out

@@ -1213,9 +1213,13 @@ ERROR:  mismatched array dimensions
 -- null key error
 select json_object('{a,b,NULL,"d e f"}','{1,2,3,"a b c"}');
 ERROR:  null value not allowed for object key
--- empty key error
+-- empty key is allowed
 select json_object('{a,b,"","d e f"}','{1,2,3,"a b c"}');
-ERROR:  empty value not allowed for object key
+                     json_object                     
+-----------------------------------------------------
+ {"a" : "1", "b" : "2", "" : "3", "d e f" : "a b c"}
+(1 row)
+
 -- json_to_record and json_to_recordset
 select * from json_to_record('{"a":1,"b":"foo","c":"bar"}')
     as x(a int, b text, d text);

src/test/regress/expected/json_1.out

@@ -1209,9 +1209,13 @@ ERROR:  mismatched array dimensions
 -- null key error
 select json_object('{a,b,NULL,"d e f"}','{1,2,3,"a b c"}');
 ERROR:  null value not allowed for object key
--- empty key error
+-- empty key is allowed
 select json_object('{a,b,"","d e f"}','{1,2,3,"a b c"}');
-ERROR:  empty value not allowed for object key
+                     json_object                     
+-----------------------------------------------------
+ {"a" : "1", "b" : "2", "" : "3", "d e f" : "a b c"}
+(1 row)
+
 -- json_to_record and json_to_recordset
 select * from json_to_record('{"a":1,"b":"foo","c":"bar"}')
     as x(a int, b text, d text);

src/test/regress/sql/json.sql

@@ -435,7 +435,7 @@ select json_object('{a,b,c,"d e f"}','{1,2,3,"a b c",g}');
 
 select json_object('{a,b,NULL,"d e f"}','{1,2,3,"a b c"}');
 
--- empty key error
+-- empty key is allowed
 
 select json_object('{a,b,"","d e f"}','{1,2,3,"a b c"}');