Fix unbound GPDB_PKG_VERSION (#27)

Author: robozmey

scripts/build-deb.sh

@@ -116,7 +116,7 @@ if [ -z ${BUILD_USER+x} ]; then
   export BUILD_USER=github
 fi
 
-if [ -z ${GPDB_PKG_VERSION} ]; then
+if [ -z "${GPDB_PKG_VERSION+x}" ]; then
   export GPDB_PKG_VERSION=${GPDB_FULL_VERSION}-${BUILD_NUMBER}-yandex.$(git --git-dir=.git rev-list HEAD --count).$(git --git-dir=.git rev-parse --short HEAD)
 fi