When extracting text from TextEquiv nodes we may encounter nodes without
index or nodes that should get sorted via the conf attribute.
Therefore we added a more complex algorithm to extract a TextEquiv and
inform the user via log messages if we encounter structures that we can
handle but may produce unexpected results.
Python's `sorted` method will fail with a TypeError when called with
`None` and Integers:
```python
>>> sorted([None, 1])
TypeError: '<' not supported between instances of 'int' and 'NoneType'
```
Therefore we are using `float('inf')` instead of `None` in case of
missing textline indices.
