Fix source greenplum_path.sh error with set -u (#11085)

The error was introduced by dc96f667.
If `set -u` was called before sourcing greenplum_path.sh with bash, an
error `ZSH_VERSION: unbound variable` would be reported.
To solve the issue, use shell syntax `{:-}` which will output an empty
value if the variable doesn't exist.

Tested with zsh, bash and dash.

gpMgmt/bin/generate-greenplum-path.sh

 #!/usr/bin/env bash
 
 cat <<"EOF"
-if test -n "$ZSH_VERSION"; then
+if test -n "${ZSH_VERSION:-}"; then
     # zsh
     SCRIPT_PATH="${(%):-%x}"
-elif test -n "$BASH_VERSION"; then
+elif test -n "${BASH_VERSION:-}"; then
     # bash
     SCRIPT_PATH="${BASH_SOURCE[0]}"
 else