message: Interpret a `split-block` as format toggle for an empty block
  (`split-block` is emitted when "return" is pressed)
  part of fixing
diff: @@ -502,12 +502,18 @@ export default class MessageComposerInput extends React.Component {
  // These are block types, not handled by RichUtils by default.
  const blockCommands = ['code-block', 'blockquote', 'unordered-list-item', 'ordered-list-item'];
  const currentBlockType = RichUtils.getCurrentBlockType(this.state.editorState);
  +
  + const shouldToggleBlockFormat = (
  + command === 'backspace' ||
  + command === 'split-block'
  + ) && currentBlockType !== 'unstyled';
  +
  if (blockCommands.includes(command)) {
  newState = RichUtils.toggleBlockType(this.state.editorState, command);
  } else if (command === 'strike') {
  // this is the only inline style not handled by Draft by default
  newState = RichUtils.toggleInlineStyle(this.state.editorState, 'STRIKETHROUGH');
  - } else if (command === 'backspace' && currentBlockType !== 'unstyled') {
  + } else if (shouldToggleBlockFormat) {
  const currentStartOffset = this.state.editorState.getSelection().getStartOffset();
  if (currentStartOffset === 0) {
  // Toggle current block type (setting it to 'unstyled')
  