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# 程式碼庫中搜尋接受 xxxOptions 且目前會將 null 替換為新 options 執行個體的多載。先檢閱所參照 pull request 的討論，然後找出每個受影響的多載及其無參數對應項目。當這些多載拒絕 null options，而無參數呼叫建構預設 options 物件時，即視為完成。