Showing help with no arguments when you have a default verb

I want to show the verbs help when you provide no arguments:

`tool` --> Shows verbs help.
`tool arg1 arg2` --> Executes default verb with those args.

By default, it either executes the default verb without arguments or, if this verb has required values, it shows the error and help for that verb.

My current workaround is this one, but I wonder if there is a neater way:

```csharp
if (args.Length == 0)
{
args = new[] { "--help" };
}
```

首先追蹤 parser 如何處理零個引數並選擇預設 verb，然後重現 issue 中的兩個命令範例。完成的標準是：不帶引數時顯示 verb 的說明，而附加引數仍會使用這些引數執行預設 verb。

由索引模型根據 Issue 內容生成。