unbound variable check makes it impossible to optionally take into account an environment variable about just

[casey]

You can also leave -u and use -z "${VAR:-}". ${VAR:-} will be empty if VAR is unset or empty, but won't trigger the unset variable check.

[laniakea64]

From man bash -

-u Treat unset variables and parameters other than the special parameters "@" and "*" as an error when
performing parameter expansion. If expansion is attempted on an unset variable or parameter, the
shell prints an error message, and, if not interactive, exits with a non-zero status.

So try removing u from the set line?

[schlichtanders]

thank you so much for your help! that is it indeed!