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>

runtime/lua/vim/_meta/vimfn.lua

@@ -4678,8 +4678,7 @@ function vim.fn.join(list, sep) end
 --- 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.
 ---
@@ -5922,7 +5921,6 @@ function vim.fn.msgpackdump(list, type) end
 ---      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.