build: fix pebble nightly ycsb benchmark

I broke it in #145118.

Epic: none
Release note: none

Author: jbowens

build/teamcity/cockroach/nightlies/pebble_nightly_ycsb_impl.sh

@@ -45,7 +45,7 @@ exit_status=$?
 # s3 bucket. If PEBBLE_SHA is set, we don't pollute the nightly benchmark
 # results with our non-master runs.
 
-if [ -z "$PEBBLE_SHA" ]; then
+if [[ -z "${PEBBLE_SHA:-}" ]]; then
   build_mkbench
 
   prepare_datadir