Detects the value of an option at the tree node.
This option is treated as `true` by default, but any trailing space
would invalidate it.

@param {node} ast
function(ast) {
      let detected = [];

      ast.traverse(function(node) {
        if (!node.is('atrulers') && !node.is('block')) return;

        if (node.length === 0 ||
            (node.length === 1 && node.first().is('space'))) {
          detected.push(false);
        }
      });

      return detected;
    }