Fix vcad_build.sh script error when not using proxy

Signed-off-by: haojia2x <haox.jiang@intel.com>

Author: haojia2x

VCAC-A/Intel_Media_Analytics_Node/scripts/vcad_build.sh

@@ -654,13 +654,14 @@ install_vcad() {
 		fi
 	fi
          #set git proxy
-         if [ ! $HTTP_PROXY ] ; then
+         if [ -z "${HTTP_PROXY-}" ] ; then
            git_proxy_flag=0
          else
            git_proxy_flag=1
-           git_http_proxy=$HTTP_PROXY
-           git_https_proxy=$HTTPS_PROXY
          fi
+	 git_https_proxy="${HTTPS_PROXY-}"
+	 git_http_proxy="${HTTP_PROXY-}"
+
 	# generate install script
 	_cd ${BUILD_DIR}
 	cat > install_package_in_image.sh <<EOF