add fallbacks in checks with '-z'

[jameslamb]

I've been spending a lot of time in this repo recently and have seen a recurring pattern.

Checks like this:

if [ -z "$1" ]; then
  echo "you have to pass an argument"
  exit 1
fi

If code like that is run when set -u is set, that informative error message and other code inside the then block will never be executed... bash will exit immediately with an error like "unbound variable ...".

This adds a fallback to null in all such checks I could find, to avoid that.

Notes for Reviewers

How I found these

git grep -E '\-n'
git grep -E '\-z'

Recent Examples

• update docs on managing CI artifacts #175 (comment)
• update handling of GitHub auth for local, interactive use #172 (comment)
• add a fallback to null in CONDA_BUILD check conda-forge/arrow-cpp-feedstock#1696

[gforsyth]

Big fan of this -- just one question on a stray commented out line

[gforsyth]