contrib/completion: bash nounset mode fixes

[scop]

- What I did

Fixed all instances I've come across that produce errors with the bash completion and the shell in nounset mode.

- How I did it

Guarded variable accesses like ${something-} instead of directly accessing $something or ${something} where the variable can be unset.

- How to verify it

Set bash in nounset mode (set -u or set -o nounset), use the bash completion. Observe errors before this PR, no errors after.

- Description for the changelog

A number of problems using the bash completion with the shell in nounset mode have been fixed.

- A picture of a cute animal (not mandatory but encouraged)

:=}

[codecov-commenter]

Codecov Report

Merging #3965 (2753057) into master (f1f12a3) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3965   +/-   ##
=======================================
  Coverage   59.20%   59.20%           
=======================================
  Files         287      287           
  Lines       24690    24690           
=======================================
  Hits        14617    14617           
  Misses       9189     9189           
  Partials      884      884           

[vvoland]

LGTM, thanks!

[albers]

LGTM, thanks for your contribution.