Agent: Use dummy username and password when testing PowerShell HTTP

The exploit_user_list and exploit_password_list are not guaranteed to
have at least one entry. If either list is empty the exploiter will
fail. Use constant strings for the username and password to avoid
potentially crashing the exploiter.

monkey/infection_monkey/exploit/powershell.py

@@ -101,8 +101,8 @@ class PowerShellExploiter(HostExploiter):
 
     def _try_ssl_login(self, use_ssl: bool):
         credentials = Credentials(
-            username=self._config.exploit_user_list[0],
-            password=self._config.exploit_password_list[0],
+            username="dummy_username",
+            password="dummy_password",
         )
 
         auth_options = AuthOptions(