Fix running simulation with no world specified on the command line

[nkoenig]

Signed-off-by: Nate Koenig [email protected]

🦟 Bug fix

Summary

Running ign gazbeo -v 4 terminates on start, when it should run an empty world.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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