dbt ls should return a zero exit code when no models are selected

[Limess]

Describe the bug

dbt ls returns an exit code of 1 when no models are selected.

Steps To Reproduce

Run dbt ls with a model selector which returns no models, e.g. my_model,config.materialized:incremental where my_model is not incremental.

Expected behavior

I'd expect a warning message, and an exit code of 0. --warn-error could be used to opt back in to the current behaviour.

Log output

The selector 'current_concepts+,config.materialized:incremental' does not match any nodes and will be ignored
No nodes selected!

JSON (separate run):

{"timestamp": "2020-11-12T18:40:54.899662Z", "message": "No nodes selected!", "channel": "dbt", "level": 13, "levelname": "WARNING", "thread_name": "MainThread", "process": 58352, "extra": {"run_state": "internal"}}

System information

Which database are you using dbt with?
n/a

The output of dbt --version:

0.18.1

The operating system you're using:

n/a

The output of python --version:

n/a

Additional context

We're using this to programatically inspect the DBT graph and would expect an empty list rather than an error here. An error means we need to either string match against the output or ignore any errors, neither of which are desirable.

Slack thread I raised this in: https://getdbt.slack.com/archives/CJARVS0RY/p1605206141123400

[jtcohen6]

Agree! I don't think it makes sense for dbt ls to ever return a nonzero exit code.