Repository.IsValid should not throw for null/empty argument

Currently `Repository.IsValid` throws an exception if the argument is null or empty. Rather than throw an exception, this method should simply return `false` in this case, similar to the behavior of [`Directory.Exists`](http://msdn.microsoft.com/en-us/library/system.io.directory.exists.aspx) or [`File.Exists`](http://msdn.microsoft.com/en-us/library/system.io.file.exists.aspx).
