infra: Fix bash comparisons

Author: kmurray

.github/kokoro/steps/vtr-test.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 
-if [ -z $VTR_TEST ]; then
+if [ -z ${VTR_TEST+x} ]; then
 	echo "Missing $$VTR_TEST value"
 	exit 1
 fi
@@ -10,7 +10,7 @@ if [ -z $MAX_CORES ]; then
 	exit 1
 fi
 
-if [ -z $VTR_TEST_OPTIONS ]; then
+if [ -z ${VTR_TEST_OPTIONS+x} ]; then
 	echo "Missing $$VTR_TEST_OPTIONS value"
 	exit 1
 fi