be more forgiving with offset that don't have atNodeEnd=true

if index is not found, it means the last position should be returned
if there is any.

We still return -1 for empty documents, as index should always point
to a valid part if positive.

src/editor/model.js

@@ -388,7 +388,11 @@ export default class EditorModel {
             currentOffset += partLen;
             return false;
         });
-        return new DocumentPosition(index, totalOffset - currentOffset);
+        if (index === -1) {
+            return this.getPositionAtEnd();
+        } else {
+            return new DocumentPosition(index, totalOffset - currentOffset);
+        }
     }
 
     /**