Presence of empty AWS_DEFAULT_PROFILE env var breaks the CLI

If `AWS_DEFAULT_PROFILE` exists in the environment and is empty:

```shell
❯ AWS_DEFAULT_PROFILE= AWS_PROFILE=not-default-one mssh ubuntu@i-aaaaaaaaaaaaa
The config profile () could not be found
```

Additionally, if `AWS_DEFAULT_PROFILE` is set to anything, the tool ignores any other profile indication and always uses the default profile.