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

src/nvim/api/private/converter.c

@@ -328,15 +328,6 @@ bool object_to_vim(Object obj, typval_T *tv, Error *err)
     for (uint32_t i = 0; i < obj.data.dictionary.size; i++) {
       KeyValuePair item = obj.data.dictionary.items[i];
       String key = item.key;
-
-      if (key.size == 0) {
-        api_set_error(err, kErrorTypeValidation,
-                      "Empty dictionary keys aren't allowed");
-        // cleanup
-        tv_dict_free(dict);
-        return false;
-      }
-
       dictitem_T *const di = tv_dict_item_alloc(key.data);
 
       if (!object_to_vim(item.value, &di->di_tv, err)) {