Handle merging None into lists/dicts

Signed-off-by: martin f. krafft <madduck@madduck.net>
diff --git a/mergers/list/base.py b/mergers/list/base.py
index a7bbc17..ea73cdd 100644
--- a/mergers/list/base.py
+++ b/mergers/list/base.py
@@ -12,5 +12,7 @@
 
     def merge(self, first, second):
         first = [first] if not isinstance(first, list) else first[:]
+        if second is None:
+            return first
         second = [second] if not isinstance(second, list) else second[:]
         return self._combine(first, second)