Fix handling of "UserKnownHostsFile none" in config

This commit fixes the handling of "none" in the UserKnownHostsFile
directive in config. Previously, setting this value to none would
usually end up causing the default known host files to be used rather
than disabling known host checking.

Thanks go to GitHub user bigpick for reporting this issue!

asyncssh/connection.py

@@ -7911,9 +7911,17 @@ def prepare(self, # type: ignore
                         rekey_seconds, connect_timeout, login_timeout,
                         keepalive_interval, keepalive_count_max)
 
-        self.known_hosts = known_hosts if known_hosts != () else \
-            (cast(List[str], config.get('UserKnownHostsFile', [])) +
-             cast(List[str], config.get('GlobalKnownHostsFile', []))) or ()
+        if known_hosts != ():
+            self.known_hosts = known_hosts
+        else:
+            user_known_hosts = \
+                cast(List[str], config.get('UserKnownHostsFile', ()))
+
+            if user_known_hosts == []:
+                self.known_hosts = None
+            else:
+                self.known_hosts = list(user_known_hosts) + \
+                    cast(List[str], config.get('GlobalKnownHostsFile', []))
 
         self.host_key_alias = \
             cast(Optional[str], host_key_alias if host_key_alias != () else

tests/test_connection.py

@@ -706,6 +706,16 @@ async def test_known_hosts_none(self):
         async with self.connect(known_hosts=None) as conn:
             self.assertEqual(conn.get_server_host_key_algs(), default_algs)
 
+    @asynctest
+    async def test_known_hosts_none_in_config(self):
+        """Test connecting with known hosts checking disabled in config file"""
+
+        with open('config', 'w') as f:
+            f.write('UserKnownHostsFile none')
+
+        async with self.connect(config='config'):
+            pass
+
     @asynctest
     async def test_known_hosts_none_without_x509(self):
         """Test connecting with known hosts checking and X.509 disabled"""