message: remove all non-first-br nodes on new empty line, not just first one
  nextSibling returned null after calling removeNode,
  so get the nextSibling first
diff: @@ -61,12 +61,13 @@ export function renderModel(editor, model) {
  let foundBR = false;
  let partNode = lineContainer.firstChild;
  while (partNode) {
  + const nextNode = partNode.nextSibling;
  if (!foundBR && partNode.tagName === "BR") {
  foundBR = true;
  } else {
  lineContainer.removeChild(partNode);
  }
  - partNode = partNode.nextSibling;
  + partNode = nextNode;
  }
  if (!foundBR) {
  lineContainer.appendChild(document.createElement("br"));
  