CI(benchmarking): fix setting LD_LIBRARY_PATH (#9191)

## Problem

`pgbench-pgvector` job from Nightly Benchmarks fails with the error:

```
/__w/_temp/f45bc2eb-4c4c-4f0a-8030-99079303fa65.sh: line 17: LD_LIBRARY_PATH: unbound variable
```

## Summary of changes
- Fix `LD_LIBRARY_PATH: unbound variable` error in benchmarks

.github/workflows/benchmarking.yml

@@ -557,7 +557,7 @@ jobs:
         ln -s /home/nonroot/pg/usr/lib/postgresql/16/bin/psql    /tmp/neon/pg_install/v16/bin/psql
         ln -s /home/nonroot/pg/usr/lib/$(uname -m)-linux-gnu     /tmp/neon/pg_install/v16/lib
 
-        LD_LIBRARY_PATH="/home/nonroot/pg/usr/lib/$(uname -m)-linux-gnu:${LD_LIBRARY_PATH}"
+        LD_LIBRARY_PATH="/home/nonroot/pg/usr/lib/$(uname -m)-linux-gnu:${LD_LIBRARY_PATH:-}"
         export LD_LIBRARY_PATH
         echo "LD_LIBRARY_PATH=${LD_LIBRARY_PATH}" >> ${GITHUB_ENV}