download-unpacked: fix strip-top-level-dir if unset and nothing should be stripped

This fixes a regression that was introduced in
commit d5f5d399.

slapos/recipe/downloadunpacked.py

@@ -91,7 +91,8 @@ class Recipe(EnvironMixin):
     strip = self._strip
     if strip is None:
       a = os.listdir(location)
-      if len(a) == 1:
+      if len(a) != 1:
+        return
       a = os.path.join(location, *a)
       if not os.path.isdir(a):
         return