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

@@ -388,8 +388,12 @@ export default class EditorModel {
             currentOffset += partLen;
             return false;
         });
+        if (index === -1) {
+            return this.getPositionAtEnd();
+        } else {
             return new DocumentPosition(index, totalOffset - currentOffset);
         }
+    }
 
     /**
      * Starts a range, which can span across multiple parts, to find and replace text.