data: support calling get() on a non-existent key, by returning None

This behavior isn't used at the moment, but is much more sensible that
the past that was to give None to whatever content-type lambda we had
(eg DefList that would call split on the None).

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>

elixir/data.py

@@ -165,8 +165,7 @@ class BsdDB:
     def get(self, key):
         key = autoBytes(key)
         p = self.db.get(key)
-        p = self.ctype(p)
-        return p
+        return self.ctype(p) if p is not None else None
 
     def get_keys(self):
         return self.db.keys()