Fix runtime error if DIB_DNF_MODULE_STREAMS is not set

Initial patch for I78d7bcf214a45245e2073428120fcbdd968e1acd
works without the envvar set, however, 'set -eu' causes it to break
if unset.

This makes the module configuration variable not required to be
set, consistent with other DIB scripts.

Change-Id: I5ca80f518d0371a18c107c061dc923876463af57
Signed-off-by: Lon Hohberger <lhh@redhat.com>

diskimage_builder/elements/rhel-common/pre-install.d/01-module-configuration

@@ -14,6 +14,8 @@ fi
 set -eu
 set -o pipefail
 
+export DIB_DNF_MODULE_STREAMS=${DIB_DNF_MODULE_STREAMS:-}
+
 if [ "${YUM}" == "dnf" ]; then
     for m in ${DIB_DNF_MODULE_STREAMS}; do
         ${YUM} -y module disable ${m/:*/}