fix(schema): do nothing when there is no series

tests/test_naming.py

@@ -31,3 +31,16 @@ def test_comic_template():
 def test_first_author_only():
     r = req(title="X", authors="Alice A, Bob B")
     assert render_template("{Author}", r) == "Alice A"
+
+
+def test_empty_series_level_is_dropped():
+    r = req(title="Der Ickabog", authors="J.K. Rowling")
+    assert render_template("{Author}/{Series}/{Title} ({Narrator})", r) == "J.K. Rowling/Der Ickabog"
+
+
+def test_series_level_kept_when_set():
+    r = req(title="Harry Potter und der Feuerkelch", authors="J.K. Rowling",
+            series="Harry Potter")
+    r.narrator = "Rufus Beck"
+    assert render_template("{Author}/{Series}/{Title} ({Narrator})", r) == \
+        "J.K. Rowling/Harry Potter/Harry Potter und der Feuerkelch (Rufus Beck)"