Fix a segmentation fault in NoStaleEvents tests

This CL fixes an issue where a sensor may not generate any events
and the processing code calls .front() on an empty std::vector.

Bug: 291779133
Test: Presubmits
Change-Id: Ibb118f239ce9ea42fc25d1d05ba6bd46a60231f0
diff --git a/sensors/aidl/vts/VtsAidlHalSensorsTargetTest.cpp b/sensors/aidl/vts/VtsAidlHalSensorsTargetTest.cpp
index 618acbb..456aee7 100644
--- a/sensors/aidl/vts/VtsAidlHalSensorsTargetTest.cpp
+++ b/sensors/aidl/vts/VtsAidlHalSensorsTargetTest.cpp
@@ -921,9 +921,15 @@
             continue;
         }
 
+        // Skip sensors with no events
+        const std::vector<Event> events = callback.getEvents(sensor.sensorHandle);
+        if (events.empty()) {
+            continue;
+        }
+
         // Ensure that the first event received is not stale by ensuring that its timestamp is
         // sufficiently different from the previous event
-        const Event newEvent = callback.getEvents(sensor.sensorHandle).front();
+        const Event newEvent = events.front();
         std::chrono::milliseconds delta =
                 duration_cast<std::chrono::milliseconds>(std::chrono::nanoseconds(
                         newEvent.timestamp - lastEventTimestampMap[sensor.sensorHandle]));
diff --git a/sensors/common/vts/2_X/VtsHalSensorsV2_XTargetTest.h b/sensors/common/vts/2_X/VtsHalSensorsV2_XTargetTest.h
index aa6e881..b381580 100644
--- a/sensors/common/vts/2_X/VtsHalSensorsV2_XTargetTest.h
+++ b/sensors/common/vts/2_X/VtsHalSensorsV2_XTargetTest.h
@@ -806,9 +806,15 @@
             continue;
         }
 
+        // Skip sensors with no events
+        const std::vector<EventType> events = callback.getEvents(sensor.sensorHandle);
+        if (events.empty()) {
+            continue;
+        }
+
         // Ensure that the first event received is not stale by ensuring that its timestamp is
         // sufficiently different from the previous event
-        const EventType newEvent = callback.getEvents(sensor.sensorHandle).front();
+        const EventType newEvent = events.front();
         milliseconds delta = duration_cast<milliseconds>(
                 nanoseconds(newEvent.timestamp - lastEventTimestampMap[sensor.sensorHandle]));
         milliseconds sensorMinDelay = duration_cast<milliseconds>(microseconds(sensor.minDelay));