code: function assertLayout(value, defaultLayout) {
    var isFalsey = require('falsey');
    if (value === false || (value && isFalsey(value))) {
      return null;
    } else if (!value || value === true) {
      return defaultLayout || null;
    } else {
      return value;
    }
  }
docs: Assert whether or not a layout should be used based on
  the given `value`. If a layout should be used, the name of the
  layout is returned, if not `null` is returned.
  
  @param  {*} `value`
  @return {String|Null} Returns `true` or `null`.
  @api private