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>

test/functional/lua/json_spec.lua

@@ -101,6 +101,8 @@ describe('vim.json.decode()', function()
     eq({['1']=2}, exec_lua([[return vim.json.decode('{"1": 2}')]]))
     eq({['1']=2, ['3']={{['4']={['5']={{}, 1}}}}},
        exec_lua([[return vim.json.decode('{"1": 2, "3": [{"4": {"5": [ [], 1]}}]}')]]))
+    -- Empty string is a valid key. #20757
+    eq({['']=42}, exec_lua([[return vim.json.decode('{"": 42}')]]))
   end)
 
   it('parses strings properly', function()
@@ -161,6 +163,8 @@ describe('vim.json.encode()', function()
   it('dumps dictionaries', function()
     eq('{}', exec_lua([[return vim.json.encode(vim.empty_dict())]]))
     eq('{"d":[]}', exec_lua([[return vim.json.encode({d={}})]]))
+    -- Empty string is a valid key. #20757
+    eq('{"":42}', exec_lua([[return vim.json.encode({['']=42})]]))
   end)
 
   it('dumps vim.NIL', function()