memory (disco): ensure that we don't cache None in disco hashes:

Add a guard to avoid mapping `None` to a hash.

libervia/backend/memory/disco.py

@@ -18,7 +18,7 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 from collections.abc import Iterable
-from typing import cast
+from typing import TYPE_CHECKING, cast
 
 from twisted.internet.interfaces import IReactorCore
 from libervia.backend import G
@@ -40,6 +40,9 @@ import time
 
 from libervia.backend.tools.utils import ensure_deferred
 
+if TYPE_CHECKING:
+    from libervia.backend.core.main import LiberviaBackend
+
 log = getLogger(__name__)
 
 
@@ -73,6 +76,10 @@ class HashManager:
         if hash_ in self.hashes:
             log.debug("ignoring hash set: it is already known")
             return
+        if disco_info is None:
+            msg = f"Cannot cache 'None' for hash {hash_}"
+            log.exception(msg)
+            raise exceptions.InternalError(msg)
         self.hashes[hash_] = disco_info
         self.persistent[hash_] = disco_info.toElement().toXml()
 
@@ -107,7 +114,7 @@ class HashManager:
 class Discovery:
     """Manage capabilities of entities"""
 
-    def __init__(self, host):
+    def __init__(self, host: "LiberviaBackend"):
         self.host = host
         # JID-based disco info cache, it is used for entities without XEP-0115 support or
         # when something goes wrong with hash validation.