Fix failed robustness job script if TEST_RC unset (#1780)

If the TEST_RC environment variable is unset, the robustness_job.sh
script exits prematurely due to an unbound variable error, instead of
starting the robustness job without that argument. This commit
evaluates said variable conditionally so that the script doesn't fail.

tools/robustness-job.sh

@@ -112,6 +112,7 @@ if [[ "${ENGINE_MODE}" = SERVER ]]; then
 fi
 
 # Source any pre-test rc files if provided
+TEST_RC="${TEST_RC:-}"
 if [[ -f ${TEST_RC} ]]; then
     source ${TEST_RC}
 fi