Wrong interpretation of xml files analysis configuration

**Describe the bug**
The configuration of xml files analysis allows to set document ids either from tags content (its text) or tag attributes. But if the tag attributes config is absent or does not contain any attribute for the document tag (even if the id really comes from a dedicated tag), then the doc id is wrongly set and it leaks into the enclosing docset tag.

**To Reproduce**
Let the doc be
```

abcdef

A text

```

This config should work:
```







```
but it does not. We have to add the useless `attributeNames` list as below:
```










```

With the config that "works", the decoded mult file ends with
```

```

while with the config that fails, we get:
```

```
The `identPrpty` should not be present in the last tag.

@benlabbe you could be interested by this information.