Fix #719: Lemmatizer can no longer output empty string

spacy/lemmatizer.py

@@ -78,7 +78,9 @@ def lemmatize(string, index, exceptions, rules):
     for old, new in rules:
         if string.endswith(old):
             form = string[:len(string) - len(old)] + new
-            if form in index or not form.isalpha():
+            if not form:
+                pass
+            elif form in index or not form.isalpha():
                 forms.append(form)
             else:
                 oov_forms.append(form)

spacy/tests/regression/test_issue719.py

@@ -4,7 +4,6 @@ from __future__ import unicode_literals
 import pytest
 
 
-@pytest.mark.xfail
 @pytest.mark.models
 @pytest.mark.parametrize('text', ["s..."])
 def test_issue719(EN, text):