fix: SpamBone regression of descr-property (#1246)

This is a regression introduced by #1227 causing descr() is called
before a session is available.

src/viur/core/bones/spam.py

@@ -46,7 +46,10 @@ def descr(self):
         """
         The descr-property is generated and uses session variables to construct the question.
         """
-        session = current.session.get()
+
+        # The session is not available during startup
+        if not (session := current.session.get()):
+            return None
 
         a = session.get("spambone.value.a")
         b = session.get("spambone.value.b")