patch 8.0.0309: cannot use an empty key in json

Problem:    Cannot use an empty key in json.
Solution:   Allow for using an empty key.

Author: brammool

src/json.c

@@ -863,7 +863,7 @@ json_decode_item(js_read_T *reader, typval_T *res, int options)
 		    && cur_item != NULL)
 	    {
 		top_item->jd_key = get_tv_string_buf_chk(cur_item, key_buf);
-		if (top_item->jd_key == NULL || *top_item->jd_key == NUL)
+		if (top_item->jd_key == NULL)
 		{
 		    clear_tv(cur_item);
 		    EMSG(_(e_invarg));

src/testdir/test_json.vim

@@ -144,6 +144,11 @@ func Test_json_decode()
   call assert_equal(type(v:none), type(json_decode('')))
   call assert_equal("", json_decode('""'))
 
+  " empty key is OK
+  call assert_equal({'': 'ok'}, json_decode('{"": "ok"}'))
+  " but not twice
+  call assert_fails("call json_decode('{\"\": \"ok\", \"\": \"bad\"}')", 'E938:')
+
   call assert_equal({'n': 1}, json_decode('{"n":1,}'))
   call assert_fails("call json_decode(\"{'n':'1',}\")", 'E474:')
   call assert_fails("call json_decode(\"'n'\")", 'E474:')

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    309,
 /**/
     308,
 /**/