hack/test/unit: use empty default values

Commit 8013d80c2 updated the hack/test/unit script to ensure that tests
are run against the right module when TESTDIRS is specified. But there's
an issue with this commit: the script has `set -u` (i.e. 'nounset'), and
some variables are set conditionally, but checked unconditionally, so it
fails.

Fix it by defining those vars to empty strings.

Signed-off-by: Albin Kerouanton <albinker@gmail.com>

hack/test/unit

@@ -16,6 +16,9 @@ BUILDFLAGS=(-tags 'netgo journald')
 TESTFLAGS+=" -test.timeout=${TIMEOUT:-5m}"
 TESTDIRS="${TESTDIRS:-./...}"
 
+: "${api_pkg_list:=}"
+: "${client_pkg_list:=}"
+
 mkdir -p bundles
 
 case "$TESTDIRS" in