Do not emit WTF if there is no delegate

WTF should only be logged when there is more than one delegates,
not when there is none.

Test: atest com.android.cts.devicepolicy.MixedDeviceOwnerTest#testKeyManagement
      and check logcat for the absence of WTF logs
Bug: 122363826

Change-Id: I51972ec4fabb2de70a6fe1700afae3aea95e34ab

services/devicepolicy/java/com/android/server/devicepolicy/DevicePolicyManagerService.java

@@ -6101,7 +6101,9 @@ public class DevicePolicyManagerService extends BaseIDevicePolicyManager {
         synchronized (getLockObject()) {
             delegates = getDelegatePackagesInternalLocked(scope, userId);
         }
-        if (delegates.size() != 1) {
+        if (delegates.size() == 0) {
+            return null;
+        } else if (delegates.size() > 1) {
             Slog.wtf(LOG_TAG, "More than one delegate holds " + scope);
             return null;
         }