Fix optionaltags filter to not error when filtering nothing.

Author: gsnedders

html5lib/filters/optionaltags.py

@@ -11,7 +11,8 @@ def slider(self):
                 yield previous2, previous1, token
             previous2 = previous1
             previous1 = token
-        yield previous2, previous1, None
+        if previous1 is not None:
+            yield previous2, previous1, None
 
     def __iter__(self):
         for previous, token, next in self.slider():

html5lib/tests/test_optionaltags_filter.py

@@ -0,0 +1,7 @@
+from __future__ import absolute_import, division, unicode_literals
+
+from html5lib.filters.optionaltags import Filter
+
+
+def test_empty():
+    assert list(Filter([])) == []