prevent zero-length removals from deleting uneditable parts

This solves an issue where, when backspacing the proceeding character next to a pill,
chrome reports the caret as being in the pill node, not at
the start of the proceeding text node. This would cause the pill
to be removed together with proceeding character.

This is a bug in any case, removing 0 characters
shouldn't remove the part

src/editor/model.js

@@ -183,6 +183,8 @@ export default class EditorModel {
             // part might be undefined here
             let part = this._parts[index];
             const amount = Math.min(len, part.text.length - offset);
+            // don't allow 0 amount deletions
+            if (amount) {
                 if (part.canEdit) {
                     const replaceWith = part.remove(offset, amount);
                     if (typeof replaceWith === "string") {
@@ -199,6 +201,9 @@ export default class EditorModel {
                     removedOffsetDecrease += offset;
                     this._removePart(index);
                 }
+            } else {
+                index += 1;
+            }
             len -= amount;
             offset = 0;
         }