fix: check for null value in normalizaDOM()

[ryicoh]

el.nextSibling is nullable. so add to check for null value.

851 |     } = _ref2;
852 |     this.normalizeBlocks(node);
853 |     node.querySelectorAll("*").forEach((el) => {
854 |       var _el$nextSibling;
855 |       if (((_el$nextSibling = el.nextSibling) === null || _el$nextSibling === void 0 ? void 0 : _el$nextSibling.nodeType) === Node.TEXT_NODE && !el.closest("pre")) {
856 |         el.nextSibling.textContent = el.nextSibling.textContent.replace(/^\n/, "");
                                              ^
TypeError: null is not an object (evaluating 'el.nextSibling.textContent')