rowsLabels property Null safety

List<List<String>> rowsLabels

The Strings describing the variation table..

If rowsLabels.length == 2, the variation table doesn't display sign of derivate. If rowsLabels.length == 3, the variation table display sign of derivate.

Add /0 in String to display a O after sign and /NAN to display a double bar for derivative sign.

The values in variation can be display on 3 levels : add /P0 for down, /P1 for middle and /P2 for up.

rowsLabels: [
                ["x", "-∞", "-1", "1", "3", "+∞"],
                ["f '(x)", "+/0", "-/NAN", "-/0","+"],
                ["f(x)", "-∞/P0", "-5/P2", "-∞/P0/+∞/P2/NAN","3/P0","+∞/P2"]


final List<List<String>> rowsLabels;