Makes test for the null command term headless (#462)

# Description

I noticed this test was failing when running in a container, because it
was not set to run in `headless` mode.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] I have run all the tests with `./orbit.sh --test` and they pass
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

source/extensions/omni.isaac.orbit/test/envs/test_null_command_term.py

@@ -10,7 +10,7 @@ from __future__ import annotations
 from omni.isaac.orbit.app import AppLauncher
 
 # launch omniverse app
-app_launcher = AppLauncher()
+app_launcher = AppLauncher(headless=True)
 simulation_app = app_launcher.app
 
 """Rest everything follows."""