Improve tab completion behavior when there are no libraries installed

[azeey]

🦟 Bug fix

Summary

If only gz-tools is installed without the other gz libraries, tab completion gives strange results because the output of ign help and ign --commands is

I cannot find any available 'ign' command:
        * Did you install any ignition library?
        * Did you set the IGN_CONFIG_PATH environment variable?
            E.g.: export IGN_CONFIG_PATH=$HOME/local/share/ignition

and that's what's used to build up the list of candidates for tab completion. This PR checks if the ign help or ign --commands ran successfully, otherwise, sets the string to "" so that it does not participate in tab completion. As a result, only the help subcommand is listed as a candidate.

This PR also gets rid of a warning message:

bash: /usr/share/gz/gz*.completion.d/*.bash_completion.sh: No such file or directory

when manually sourcing the <prefix>/share/bash-completion/completions/ign script with no other gz libraries installed.

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)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

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