patch 8.0.1391: encoding empty string to JSON sometimes gives "null"

Problem:    Encoding empty string to JSON sometimes gives "null".
Solution:   Handle NULL string as empty string. (closes #2446)

Author: brammool

src/json.c

@@ -92,7 +92,7 @@ write_string(garray_T *gap, char_u *str)
     char_u	numbuf[NUMBUFLEN];
 
     if (res == NULL)
-	ga_concat(gap, (char_u *)"null");
+	ga_concat(gap, (char_u *)"\"\"");
     else
     {
 #if defined(FEAT_MBYTE) && defined(USE_ICONV)
@@ -237,7 +237,7 @@ json_encode_item(garray_T *gap, typval_T *val, int copyID, int options)
 	case VAR_LIST:
 	    l = val->vval.v_list;
 	    if (l == NULL)
-		ga_concat(gap, (char_u *)"null");
+		ga_concat(gap, (char_u *)"[]");
 	    else
 	    {
 		if (l->lv_copyID == copyID)
@@ -272,7 +272,7 @@ json_encode_item(garray_T *gap, typval_T *val, int copyID, int options)
 	case VAR_DICT:
 	    d = val->vval.v_dict;
 	    if (d == NULL)
-		ga_concat(gap, (char_u *)"null");
+		ga_concat(gap, (char_u *)"{}");
 	    else
 	    {
 		if (d->dv_copyID == copyID)

src/testdir/test_json.vim

@@ -103,6 +103,10 @@ func Test_json_encode()
   call assert_fails('echo json_encode(function("tr"))', 'E474:')
   call assert_fails('echo json_encode([function("tr")])', 'E474:')
 
+  call assert_equal('{"a":""}', json_encode({'a': test_null_string()}))
+  call assert_equal('{"a":[]}', json_encode({"a": test_null_list()}))
+  call assert_equal('{"a":{}}', json_encode({"a": test_null_dict()}))
+
   silent! let res = json_encode(function("tr"))
   call assert_equal("", res)
 endfunc

src/version.c

@@ -771,6 +771,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1391,
 /**/
     1390,
 /**/