bash-completion: make completions to work when bash set -u is in use

bash-completion: make completions to work when bash set -u is in use

User who want to avoid refering to none-existing variables got earlier the
following error.

$ set -u
$ findmnt --output <tab>bash: OUTPUT: unbound variable

Here is short explanation of this setting.

$ help set
 -u  Treat unset variables as an error when substituting.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>