function wfHeadingsBlank(ast, file, setting) {
  visit(ast, 'heading', function(node) {
    let title = toString(node).trim();
    if (title.length === 0) {
      file.message('Headings cannot be empty.', node);
    }
  });
}
Remark Lint Test - flags empty headings.

@param {Object} ast
@param {Object} file
@param {Object} setting