Set terminals_available to False when not enabled

[Wh1isper]

Suggested by @lp9052 in #85 (comment)

by looking at the source code, it does respect the terminal config.

However, I believe it should update termina_avaliable to False instead of just return.

nbclassic actually read this field and will fail to launch if the value is missing.

I know they should have a default value for missing fields(issue raised). But I also believe it should be populated, instead of just missing.

Link to nbclassic issue: jupyter/nbclassic#255

this allows us to be consistent with Jupyter Server 1.x behaviour: https://github.com/jupyter-server/jupyter_server/blob/1.x/jupyter_server/serverapp.py#L363

[Wh1isper]

@blink1073 Can you take a look at your convenience?

[blink1073]

Thank you!