utils/ssh: Only attempt loading ssh keys if no password is supplied

In the case of connecting to a system without a password the password
parameter needs to be set to an empty string which will currently still
cause ssh keys to be loaded. Only check for ssh keys when the password
is explicitly set to `None`.

devlib/utils/ssh.py

@@ -351,7 +351,7 @@ class SshConnection(SshConnectionBase):
         else:
             policy = AutoAddPolicy
         # Only try using SSH keys if we're not using a password
-        check_ssh_keys = not self.password
+        check_ssh_keys = self.password is None
 
         with _handle_paramiko_exceptions():
             client = SSHClient()