Stop leaking 'set()' into bash completion script

[t184256]

Mirroring xonsh/xonsh#2814

Using set() as a fallback for source is quite odd,

py-bash-completion/bash_completion.py

Line 297 in 90c9d08

source = _get_bash_completions_source(paths) or set()

as it ultimately leaks to the bash completions file as set()

py-bash-completion/bash_completion.py

Line 199 in 90c9d08

{source}

and the ensuing error gets swallowed at

py-bash-completion/bash_completion.py

Line 333 in 90c9d08

except (subprocess.CalledProcessError, FileNotFoundError,

[scopatz]

Thanks for opening this here @t184256! That set() should probably be an empty string now.

[scopatz]

Fix available in #12

[t184256]

Does it make sense to run the bash script at all in that case?

[scopatz]

It might... But at least this won't fail