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);
     [...]
}

Search the C# codebase for overloads that accept xxxOptions and currently replace null with a new options instance. Review the referenced pull request discussion first, then identify every affected overload and its parameterless counterpart. Done means these overloads reject null options while parameterless calls construct the default options object.