keys: add an in-memory key-store fallback

Instead of defaulting the key-store to `None`, use an in-memory one.
This way a keystore is always available. A future improvement could be
to rework the device keystore initialization to remove checks like
`if self.keystore:` along the codebase.

bumble/device.py

@@ -2243,6 +2243,7 @@ class Device(CompositeEventEmitter):
                     return None
 
                 return keys.link_key.value
+        return None
 
     # [Classic only]
     async def authenticate(self, connection):

bumble/keys.py

@@ -25,7 +25,7 @@ import asyncio
 import logging
 import os
 import json
-from typing import TYPE_CHECKING, Optional
+from typing import TYPE_CHECKING, Dict, List, Optional, Tuple
 
 from .colors import color
 from .hci import Address
@@ -139,19 +139,19 @@ class PairingKeys:
 
 # -----------------------------------------------------------------------------
 class KeyStore:
-    async def delete(self, name):
+    async def delete(self, name: str):
         pass
 
-    async def update(self, name, keys):
+    async def update(self, name: str, keys: PairingKeys) -> None:
         pass
 
-    async def get(self, _name):
+    async def get(self, _name: str) -> Optional[PairingKeys]:
-        return PairingKeys()
+        return None
 
-    async def get_all(self):
+    async def get_all(self) -> List[Tuple[str, PairingKeys]]:
         return []
 
-    async def delete_all(self):
+    async def delete_all(self) -> None:
         all_keys = await self.get_all()
         await asyncio.gather(*(self.delete(name) for (name, _) in all_keys))
 
@@ -177,15 +177,15 @@ class KeyStore:
             separator = '\n'
 
     @staticmethod
-    def create_for_device(device: Device) -> Optional[KeyStore]:
+    def create_for_device(device: Device) -> KeyStore:
         if device.config.keystore is None:
-            return None
+            return MemoryKeyStore()
 
         keystore_type = device.config.keystore.split(':', 1)[0]
         if keystore_type == 'JsonKeyStore':
             return JsonKeyStore.from_device(device)
 
-        return None
+        return MemoryKeyStore()
 
 
 # -----------------------------------------------------------------------------
@@ -307,3 +307,24 @@ class JsonKeyStore(KeyStore):
             return None
 
         return PairingKeys.from_dict(keys)
+
+
+# -----------------------------------------------------------------------------
+class MemoryKeyStore(KeyStore):
+    all_keys: Dict[str, PairingKeys]
+
+    def __init__(self) -> None:
+        self.all_keys = {}
+
+    async def delete(self, name: str) -> None:
+        if name in self.all_keys:
+            del self.all_keys[name]
+
+    async def update(self, name: str, keys: PairingKeys) -> None:
+        self.all_keys[name] = keys
+
+    async def get(self, name: str) -> Optional[PairingKeys]:
+        return self.all_keys.get(name)
+
+    async def get_all(self) -> List[Tuple[str, PairingKeys]]:
+        return list(self.all_keys.items())