message: also process first part when processing empty and mergeable parts
  this was preventing clearing an emtpy plain part when inserting
  a pill-candidate at the beginning of the model, which
  prevented a caret node from being inserted before the pill.
diff: @@ -158,11 +158,11 @@ export default class EditorModel {
  }
  _mergeAdjacentParts(docPos) {
  - let prevPart = this._parts[0];
  - for (let i = 1; i < this._parts.length; ++i) {
  + let prevPart;
  + for (let i = 0; i < this._parts.length; ++i) {
  let part = this._parts[i];
  const isEmpty = !part.text.length;
  - const isMerged = !isEmpty && prevPart.merge(part);
  + const isMerged = !isEmpty && prevPart && prevPart.merge(part);
  if (isEmpty || isMerged) {
  // remove empty or merged part
  part = prevPart;
  