An empty threat model with ignoreUnused throws an error

[raphaelahrens]

If a model has only elements which will not be displayed because
ignoreUnused is set to True the function _sort_elements() fails.

This is caused by a call to max(orders.values())which then ismax([])` which is not a computable value.

The error is caused by a module like this or an empty model.

from pytm import (
    TM,
    Actor,
)

tm = TM("my test tm")
tm.ignoreUnused = True

actor = Actor('actor')

if __name__ == "__main__":
    tm.process()

[nineinchnick]

I'd say this should still raise an error, just with a more actionable message.

[raphaelahrens]

I am not sure if the check is really sufficient, maybe it can be done in the call to max() directly.
For example `max([-1]+orders.values())

[raphaelahrens]

I'd say this should still raise an error, just with a more actionable message.

Should an empty model also raise such an error like the following?

tm = TM("my test tm")
tm.ignoreUnused = True

if __name__ == "__main__":
    tm.process()

Because this model is not raising any errors.

tm = TM("my test tm")

if __name__ == "__main__":
    tm.process()

[izar]

Thanks!