Use something better than null for isolation status

[edongashi]

We get a null when isolation status is skipped. This is confusing and causes bugs.

[sebastian]

For clarification: you mean in the code that needs to know whether a column is isolating or not inside the cloak, or outside the cloak in say air.

[edongashi]

Air needs to know isolation status in some places.

1. SHOW COLUMNS FROM ....
2. The UI column background color and label.

When we say a column has isolation_status: null (and send it through the wire from cloak to air) it is very unclear what is meant by that.