Fix crash when PYTORCH_VERSION is not defined (#455)

Author: atalman

packaging/pre_build_script.sh

@@ -10,7 +10,7 @@ set -eux
 echo "This script is run before building torchao binaries"
 
 python -m pip install --upgrade pip
-if [ -z "$PYTORCH_VERSION" ]; then
+if [ -z ${PYTORCH_VERSION:-} ]; then
     PYTORCH_DEP="torch"
 else
     PYTORCH_DEP="torch==$PYTORCH_VERSION"