fix(defaults): omit empty line from unimpaired mapping messages (#31347)

Problem:
The default unimpaired mappings display an empty line after the
command's output. This results (with default configuration) in the
`Press ENTER or type command to continue` prompt to be displayed, like
so:
```
(2 of 16): item2

Press ENTER or type command to continue
```

Solution:
The cause is that we're checking the second return value from
`pcall(vim.api.nvim_cmd, opts, {})` to determine whether the call was
successful. `nvim_cmd` returns an empty string on success, so this value
is an empty string in the successful path which we then display.

The fix is simple: check the first return value instead which is the
"status code" of the call.

runtime/lua/vim/_defaults.lua

@@ -222,8 +222,8 @@ do
     --- Execute a command and print errors without a stacktrace.
     --- @param opts table Arguments to |nvim_cmd()|
     local function cmd(opts)
-      local _, err = pcall(vim.api.nvim_cmd, opts, {})
-      if err then
+      local ok, err = pcall(vim.api.nvim_cmd, opts, {})
+      if not ok then
         vim.api.nvim_err_writeln(err:sub(#'Vim:' + 1))
       end
     end