FirstOrDefault() method doesn't work when database splitting is used

[AlexandrSHad]

Looks like FirstOrDefault() method doesn't work in combination with a discriminator.
To reproduce this you can add the following tests to the file:

        [Fact]
        public void Discriminator_FirstOrDefault()
        {
            var json1 = _rebels.FirstOrDefault().ToString();
            var json2 = _simpleRebels.FirstOrDefault().ToString();
            Assert.Equal(@"{""limit"":1, ""selector"":{""split_discriminator"":""Rebel""}}", json1);
            Assert.Equal(@"{""limit"":1, ""selector"":{""split_discriminator"":""SimpleRebel""}}", json2);
        }

        [Fact]
        public void Discriminator_FirstOrDefault_WithExpression()
        {
            var json1 = _rebels.FirstOrDefault(c => c.Age == 19).ToString();
            var json2 = _simpleRebels.FirstOrDefault(c => c.Age == 19).ToString();
            Assert.Equal(@"{""limit"":1, ""selector"":{""$and"":[{""age"":19},{""split_discriminator"":""Rebel""}]}}", json1);
            Assert.Equal(@"{""limit"":1, ""selector"":{""$and"":[{""age"":19},{""split_discriminator"":""SimpleRebel""}]}}", json2);
        }

Exception "System.IndexOutOfRangeException : Index was outside the bounds of the array." will be thrown.