$ cat t.sh
#=!/bin/sh
set -x
FETCH_HEAD_OR_EMPTY=$([ -z "${1:-}" ] || echo FETCH_HEAD)
printf 'HERE %s\n' "${FETCH_HEAD_OR_EMPTY}"
FETCH_HEAD_OR_EMPTY=${1:-FETCH_HEAD}
printf 'HERE %s\n' "${FETCH_HEAD_OR_EMPTY}"
$ ./t.sh 
+ [ -z  ]
+ FETCH_HEAD_OR_EMPTY=
+ printf HERE %s\n 
HERE 
+ FETCH_HEAD_OR_EMPTY=FETCH_HEAD
+ printf HERE %s\n FETCH_HEAD
HERE FETCH_HEAD
$ ./t.sh foo
+ [ -z foo ]
+ echo FETCH_HEAD
+ FETCH_HEAD_OR_EMPTY=FETCH_HEAD
+ printf HERE %s\n FETCH_HEAD
HERE FETCH_HEAD
+ FETCH_HEAD_OR_EMPTY=foo
+ printf HERE %s\n foo
HERE foo