Fix some issues with setting null values to fields.

[pnomolos]

I've come across some further issues when dealing with nulls, both due to how isset handles null fields and with the fact that null is the same as 0 or an empty string with non-strict comparison. There's one test in the test suite that will fail until my earlier fix for the Integer type-casting is merged as well.

[vlucas]

Merged in with a cherry-pick. SHA1: 53b91c055d9d0572495595f34639b48bc215a6c3