Bugfix with undefined children_hooks when package is None

touch_import_top() is always called with a package within
the futurize codebase, but it does have a code path where
package can be None (e.g. import six instead of from six
import string_types). Fixed a minor bug in this code path,
so that a custom fixer can use it.

Change-Id: Iec2891586fe852e35a91c69d2fb146645d7c53dd

Author: azjps

src/libfuturize/fixer_util.py

@@ -390,6 +390,7 @@ def touch_import_top(package, name_to_import, node):
                 break
         insert_pos = idx
 
+    children_hooks = []
     if package is None:
         import_ = Node(syms.import_name, [
             Leaf(token.NAME, u"import"),
@@ -413,8 +414,6 @@ def touch_import_top(package, name_to_import, node):
                                  ]
                                 )
             children_hooks = [install_hooks, Newline()]
-        else:
-            children_hooks = []
 
         # FromImport(package, [Leaf(token.NAME, name_to_import, prefix=u" ")])