Throw when xxxOptions are null

From https://github.com/libgit2/libgit2sharp/pull/1068/files#r31650592

We rely on the following pattern throughout the codebase.

public Things DoThings()
{
     return DoThings(null);
}

public Things DoThings(ThingsOptions options)
{
     options = options ?? new ThingsOptions();
     [...]
}

Let's change this to the following one

public Things DoThings()
{
     return DoThings(new ThingsOptions());
}

public Things DoThings(ThingsOptions options)
{
     Ensure.ArgumentNotNull(options);
     [...]
}

在 C# 代码库中搜索接受 xxxOptions 且当前会将 null 替换为新 options 实例的重载。先查看所引用 pull request 的讨论，然后确定每个受影响的重载及其无参数对应项。当这些重载拒绝 null options，而无参数调用构造默认 options 对象时，即视为完成。