Add quotes in empty check.

Required by bash.

Test: run-gtests.sh
Change-Id: Ibecb82066ab9e9ee68b7014dd54d503823b8b17b
diff --git a/tools/run-gtests.sh b/tools/run-gtests.sh
index bf29023..8585589 100755
--- a/tools/run-gtests.sh
+++ b/tools/run-gtests.sh
@@ -35,7 +35,7 @@
   ${ADB} shell "chroot $ART_TEST_CHROOT env LD_LIBRARY_PATH= ANDROID_ROOT='/system' ANDROID_RUNTIME_ROOT=/system $i" || fail $i
 done
 
-if [ -n $failing_tests ]; then
+if [ -n "$failing_tests" ]; then
   for i in "${failing_tests[@]}"; do
     echo "Failed test: $i"
   done