Sort textlines with missing indices

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.

qurator/dinglehopper/extracted_text.py

@@ -194,7 +194,7 @@ class ExtractedText:
                 try:
                     return int(index)
                 except TypeError:
-                    return None
+                    return float('inf')
             textequivs = sorted(textequivs, key=get_index)
 
             return textequivs[0].find('./page:Unicode', namespaces=nsmap).text

qurator/dinglehopper/tests/extracted_text_test.py

@@ -93,3 +93,26 @@ def test_textequiv_index():
     expected = "gefahren zu haben, einzelne Bemerkungen und Beobach-"
 
     assert expected == result
+
+
+def test_textequiv_no_index():
+    """
+    Test that extracting text from a PAGE TextEquiv ignores missing indices.
+    """
+
+    textline="""<?xml version="1.0"?>
+      <TextLine id="l3" xmlns="http://schema.primaresearch.org/PAGE/gts/pagecontent/2018-07-15">
+        <TextEquiv>
+          <Unicode>gefahren zu haben, einzelne Bemorkungen und Beobäch-</Unicode>
+        </TextEquiv>
+        <TextEquiv index="1">
+          <Unicode>gefahren zu haben, einzelne Bemerkungen und Beobach-</Unicode>
+        </TextEquiv>
+      </TextLine>
+    """
+    root = ET.fromstring(textline)
+    nsmap = {'page': "http://schema.primaresearch.org/PAGE/gts/pagecontent/2018-07-15" }
+    result = ExtractedText.from_text_segment(root, nsmap, textequiv_level='line').text
+    expected = "gefahren zu haben, einzelne Bemerkungen und Beobach-"
+
+    assert expected == result