feat(rpc): allow empty string key in msgpack => Vim conversion

Problem:
Since e057b38e70 #20757 we support empty key in JSON encode/decode,
but we don't allow it in RPC object => Vim dict conversion. But empty
string is a valid key in Vim dicts and the msgpack spec.

Empty string key was disallowed in 7c01d5ff92 (2014) but that
commit/PR doesn't explicitly discuss it, so presumably it was a "seems
reasonable" decision (or Vimscript didn't allow empty keys until later).

Solution:
Remove the check in `object_to_vim()`. Note that
`tv_dict_item_alloc_len` will invoke `memcpy(…, 0)` but that's allowed
by the C spec: https://stackoverflow.com/a/3751937/152142

test/functional/vimscript/json_functions_spec.lua

@@ -576,8 +576,8 @@ describe('json_encode() function', function()
     eq('{}', eval('json_encode({})'))
     eq('{"d": []}', funcs.json_encode({d={}}))
     eq('{"d": [], "e": []}', funcs.json_encode({d={}, e={}}))
-    -- Empty keys not allowed (yet?) in object_to_vim() (since 7c01d5ff9286). #25564
-    -- eq('{"": []}', funcs.json_encode({['']={}}))
+    -- Empty keys are allowed per JSON spec (and Vim dicts, and msgpack).
+    eq('{"": []}', funcs.json_encode({['']={}}))
   end)
 
   it('cannot dump generic mapping with generic mapping keys and values',