Will Dispose() be called in a using statement with a null object?

Yes, Dispose() is only called on non-null objects:

http://msdn.microsoft.com/en-us/library/yh598w02.aspx

The expansion for using checks that the object is not null before calling Dispose on it, so yes, it's safe.

In your case you would get something like:

IDisposable x = GetObject("invalid name");
try
{
    // etc...
}
finally
{
    if(x != null)
    {
        x.Dispose();
    }
}