Use test -n to check the existence of CUDA_VISIBLE_DEVICES (#1204)

With `set -u`, if CUDA_VISIBLE_DEVICES is not set outside of
`test-maxtext.sh`, then `if [ "$CUDA_VISIBLE_DEVICES" != "" ]; then`
will introduce `-bash: CUDA_VISIBLE_DEVICES: unbound variable` error.

.github/container/test-maxtext.sh

@@ -174,7 +174,7 @@ if [ $DTYPE == "fp8" ]; then
 fi
 
 GPUS_PER_NODE=$(nvidia-smi -L | grep -c '^GPU')
-if [ "$CUDA_VISIBLE_DEVICES" != "" ]; then
+if [ -n "${CUDA_VISIBLE_DEVICES:-}" ]; then
   GPUS_PER_NODE=`python -c 'import os; x=os.environ.get("CUDA_VISIBLE_DEVICES", ""); print(len(x.split(",")))'`
 fi
 NGPUS=$((GPUS_PER_NODE * NODES))