fix(json): allow objects with empty keys #25564

Problem:
Empty string is a valid JSON key, but json_decode() treats an object
with empty key as ":help msgpack-special-dict". #20757

    :echo json_decode('{"": "1"}')
    {'_TYPE': [], '_VAL': [['', '1']]}

Note: vim returns `{'': '1'}`.

Solution:
Allow empty string as an object key.

Note that we still (currently) disallow empty keys in object_to_vim() (since 7c01d5ff92):
f64e4b43e1/src/nvim/api/private/converter.c (L333-L334)

Fix #20757

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

src/nvim/eval.lua

@@ -5735,8 +5735,7 @@ M.funcs = {
       Vim value. In the following cases it will output
       |msgpack-special-dict|:
       1. Dictionary contains duplicate key.
-      2. Dictionary contains empty key.
-      3. String contains NUL byte.  Two special dictionaries: for
+      2. String contains NUL byte.  Two special dictionaries: for
          dictionary and for string will be emitted in case string
          with NUL byte was a dictionary key.
 
@@ -7155,7 +7154,6 @@ M.funcs = {
       	   are binary strings).
       	2. String with NUL byte inside.
       	3. Duplicate key.
-      	4. Empty key.
       ext	|List| with two values: first is a signed integer
       	representing extension type. Second is
       	|readfile()|-style list of strings.