Deleting a server that has no environment variables raises sqlalchemy exception

[vilhelmprytz]

Environment

• Installation type: snap and pip
• Python version: 3.7.3
• Wilfred version: v3.0.0

Steps to Reproduce

1. Create a server that does not require any environment variables (such as minecraft-bungeecord).
2. Try to delete it using wilfred delete <name>.

Expected Behavior

Server to be deleted without exceptions.

Observed Behavior

Raises sqlalchemy exception, it tries to delete environment variables which do not exist. Wilfred does not handle this.

See traceback below.

Are you sure you want to do this? All data will be permanently deleted. [y/N]: y
Traceback (most recent call last):
  File "/home/vilhelm/.local/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 1999, in delete
    state = attributes.instance_state(instance)
AttributeError: 'NoneType' object has no attribute '_sa_instance_state'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/bin/wilfred", line 10, in <module>
    sys.exit(main())
  File "/home/vilhelm/.local/lib/python3.7/site-packages/wilfred/wilfred.py", line 88, in main
    cli()
  File "/home/vilhelm/.local/lib/python3.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/vilhelm/.local/lib/python3.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/vilhelm/.local/lib/python3.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/vilhelm/.local/lib/python3.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/vilhelm/.local/lib/python3.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/vilhelm/.local/lib/python3.7/site-packages/wilfred/wilfred.py", line 409, in delete
    servers.remove(server)
  File "/home/vilhelm/.local/lib/python3.7/site-packages/wilfred/servers.py", line 116, in remove
    session.query(EnvironmentVariable).filter_by(server_id=server.id).first()
  File "/home/vilhelm/.local/lib/python3.7/site-packages/sqlalchemy/orm/session.py", line 2001, in delete
    raise exc.UnmappedInstanceError(instance)
sqlalchemy.orm.exc.UnmappedInstanceError: Class 'builtins.NoneType' is not mapped