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/api/vim_spec.lua

@@ -1319,6 +1319,10 @@ describe('API', function()
       eq("Key not found: lua", pcall_err(meths.del_var, 'lua'))
       meths.set_var('lua', 1)
 
+      -- Empty keys are allowed in Vim dicts (and msgpack).
+      nvim('set_var', 'dict_empty_key', {[''] = 'empty key'})
+      eq({[''] = 'empty key'}, nvim('get_var', 'dict_empty_key'))
+
       -- Set locked g: var.
       command('lockvar lua')
       eq('Key is locked: lua', pcall_err(meths.del_var, 'lua'))
@@ -1983,7 +1987,7 @@ describe('API', function()
     it('errors when context dictionary is invalid', function()
       eq('E474: Failed to convert list to msgpack string buffer',
          pcall_err(nvim, 'load_context', { regs = { {} }, jumps = { {} } }))
-      eq("Empty dictionary keys aren't allowed",
+      eq('E474: Failed to convert list to msgpack string buffer',
          pcall_err(nvim, 'load_context', { regs = { { [''] = '' } } }))
     end)
   end)