onecmd doesn't work without full session

[theguly]

running weevely in the "onecmd" mode as per https://github.com/epinna/weevely3/blob/master/weevely.py#L59 doesn't run anything if the session has not been established beforehands.

i guess the self.default() called at https://github.com/epinna/weevely3/blob/0bae932d5cb0e96370e1b76fbdb6ca84bd42c0ed/core/terminal.py#L106 after no do_ or do_alias_ commands are found doesn't support an session with empty default_shell

to reproduce:

• clean up all sessions: rm -rf ~/.weevely/sessions
• add a webshell to a remote target
• run a single command: weevely.py http://127.0.0.1/weevely.php password "ls"
• observe that no output is returned

now force a shell_sh:

• run a single command: weevely.py http://127.0.0.1/weevely.php password "shell_sh ls"
• observe that shell_sh is executed and returns expected output

open a session and run a single command:

• weevely.py http://127.0.0.1/weevely.php password
• run any command to fix default_shell at session file: weevely> ls
• exit weevely> ctrl+c
• weevely.py http://127.0.0.1/weevely.php password "ls"
• observe that ls command now returns expected output