Make it work with set -u

[hoijui]

Intro

From the man page of set in BASH:

-u Treat unset variables as an error when substituting.

In other words - like many of the other set parameters - this allows a script to fail-fast.
Using when editing the script in a modern editor/IDE, also enables linting for those errors.

What it's changing and/or adding?

Where $1 and $2 are used and they might be unset, this change replaces them with ${1:-} and ${2:-},
which means "use $1 if set, else the empty string"

What OS and/or Bash version it happens? (fix only)

All, if set -u is used.