Can you please try this with the latest unstable?
var test = s.Advanced.LuceneQuery<Dummy>("Temp/Dummies/ByBooleanAndObject") .Where("Boolean:true OR (-Object:[[NULL_VALUE]] AND Object:*)") .ToList();
Assert.Equal(3, test.Count);
Note the brackets around the ".. AND .." statement to change the precedence. I think this is a bug in RavenDB as it's not generating the right query