Allow rows that are null to be valid existing values

[dhensby]

What this does:
fixes #1161

todo - decide if this is a breaking change.

Possibly this means there's no need for arrayRowMode?!

[dhensby]

I'm a little cautious about introducing this into the v7 branch given it's a fairly significant (though intuitive) behaviour change. I'm also worried that it may introduce some regressions in other queries that you wouldn't expect it in.

[mtriff]

Just chiming in that arrayRowMode is still needed in order to re-construct the results in the order specified in the query. For example, SELECT t1.a, t1.b, t2.a FROM table1 t1 JOIN table2 t2. You need arrayRowMode (or something else like it) to know the order a, b, a without parsing the query.