I have tested it several times in SQL Server 2000 and 2005. Every time, it gives NULL first. I guess that it shows the NULL first because NULL has no values. Then it shows the values.
It's great that testing shows consistent results, but that doesn't make this the "only" correct answer. A query without ORDER BY returns a relational set, which (by the very definitions of the relational model) has no inherent order. Limitations of the clients force some order upon that set, but unless an ORDER BY clause is specified, that order is indetermined and prone to change at SQL Server's whim.
Undocumented behaviour, even if confirmed by repeated testing, should never be relied on. I thought that after the "GROUP BY no longer implies sorting" change back in 6.5 and the "TOP 100 PERCENT ... ORDER BY in views no longer orders the view" change in 2005, that should be clear by now!
Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: http://sqlblog.com/blogs/hugo_kornelis