Bug fix: null string will cause exception in checkStringInAdbCommandOutput

Bug: 192036015
Test: atest com.android.settings.fuelgauge.batterysaver.BatterySaverButtonPreferenceControllerComponentTest#test_check_battery_saver_button
Change-Id: Ifc73c8c9d6b5fabca6af5ea604f23a789b99d217
https://atp.googleplex.com/test_runs/128222905

tests/componenttests/src/com/android/settings/testutils/AdbUtils.java

@@ -43,10 +43,12 @@ public class AdbUtils {
     public static boolean checkStringInAdbCommandOutput(String logTag, String command,
             String prefix, String target, int timeoutInMillis) throws Exception {
         long start = System.nanoTime();
+
         //Sometimes the change do no reflect in adn output immediately, so need a wait and poll here
         while (System.nanoTime() - start < (timeoutInMillis * 1000000)) {
             String result = shell(command);
-            if (result.contains(prefix) && result.contains(target)) {
+            if (result.contains(prefix == null ? "" : prefix)
+                    && result.contains(target == null ? "" : target)) {
                 return true;
             } else {
                 Thread.sleep(100);