Added empty expression to line generator.

tests/test_line.py

@@ -6,11 +6,8 @@ from line import generate_line
 
 class TestLine(unittest.TestCase):
 
-    def setUp(self):
-        pass
-
-    def tearDown(self):
-        pass
+    def test_empty(self):
+        self.assertEquals(generate_line(None), '<empty expression>')
 
     def test_simple(self):
         l0, l1 = L(1), L(2)