Do not show exception w/ file completer when pressing tab after backslash

IPython/completer.py

@@ -380,7 +380,7 b' class IPCompleter(Completer):'
                 open_quotes = 1
                 lsplit = lbuf.split("'")[-1]
             else:
-                return None
+                return []
         except IndexError:
             # tab pressed on empty line
             lsplit = ""