IMPALA-11873: Fix unbound variable of OZONE_ERASURECODE_POLICY when E…

…C is diabled

This fixes a script error of unbound variable when running the mini
cluster on Ozone with ERASURE_CODING=false. In such case,
OZONE_ERASURECODE_POLICY is not set in bin/impala-config.sh. Adds a
default empty string when using it.

Tests:
 - Verified CORE tests can run with TARGET_FILESYSTEM=ozone and
   ERASURE_CODING=false.

Change-Id: If4c50653ec7e317c3246055adc50d4dd99272173
Reviewed-on: http://gerrit.cloudera.org:8080/19455
Reviewed-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>

testdata/cluster/admin

@@ -329,7 +329,7 @@ function start_cluster {
   if [[ "${TARGET_FILESYSTEM}" = "ozone" ]]; then
     local replication=''
     echo "Creating Ozone volume/bucket"
-    if [ -n "${OZONE_ERASURECODE_POLICY}" ]; then
+    if [ -n "${OZONE_ERASURECODE_POLICY:-}" ]; then
       replication="--type EC --replication ${OZONE_ERASURECODE_POLICY}"
       echo "with erasure coding ${OZONE_ERASURECODE_POLICY}"
     fi