bash `nounset` mode fixes

[scop]

Fixes issues I came across while briefly testing the shtab completions of keyring in set -o nounset mode, as well tests failing in that mode.

This might not be a complete set, but it's a start.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (a6f5ba9) 88.88% compared to head (e6a5042) 88.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #143 +/- ## ======================================= Coverage 88.88% 88.88% ======================================= Files 3 3 Lines 360 360 ======================================= Hits 320 320 Misses 40 40 ``` | [Files Changed](https://app.codecov.io/gh/iterative/shtab/pull/143?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iterative) | Coverage Δ | | |---|---|---| | [shtab/\_\_init\_\_.py](https://app.codecov.io/gh/iterative/shtab/pull/143?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iterative#diff-c2h0YWIvX19pbml0X18ucHk=) | `90.76% <ø> (ø)` | |

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