thermal: Add checks to search for thermal zones first

There are instances where PM-QA might be running on
systems that do not have thermal zones and might have
cooling systems instead; therefore some subtests may
instantly fail because of it. Add more checks to see
if thermal zones exist before continuing the thermal
subtests.

Signed-off-by: Lisa Nguyen <lisa.nguyen@linaro.org>
diff --git a/thermal/thermal_06.sh b/thermal/thermal_06.sh
index 9c744f1..5774977 100755
--- a/thermal/thermal_06.sh
+++ b/thermal/thermal_06.sh
@@ -109,5 +109,9 @@
 
 trap "heater_kill; sigtrap" HUP INT TERM
 
-for_each_thermal_zone check_trip_point_change
+if [ -z "$thermal_zones"]; then
+   log_skip "No thermal zones found"
+else
+    for_each_thermal_zone check_trip_point_change
+fi
 test_status_show