Try to more safely handle empty hostnames.

Test: as follows
    - built
    - flashed
    - booted
    - manual testing in modes: {off, opportunistic, strict}
    - system/netd/tests/runtests.sh
Bug: 74606156
Change-Id: I183e5de0ba77571fefd686ac0a9e53e6011722a4
diff --git a/server/ResolverController.cpp b/server/ResolverController.cpp
index a5074e2..24c996d 100644
--- a/server/ResolverController.cpp
+++ b/server/ResolverController.cpp
@@ -134,9 +134,9 @@
             netdEventListener = eventReporter.getNetdEventListener();
         }
         if (netdEventListener != nullptr) {
-            netdEventListener->onPrivateDnsValidationEvent(netId,
-                    String16(addrToString(&(server.ss)).c_str()),
-                    String16(server.name.c_str()), success);
+            const String16 ipLiteral(addrToString(&(server.ss)).c_str());
+            const String16 hostname(server.name.empty() ? "" : server.name.c_str());
+            netdEventListener->onPrivateDnsValidationEvent(netId, ipLiteral, hostname, success);
             if (DBG) {
                 ALOGD("Sending validation %s event on netId %u for %s with hostname %s",
                         success ? "success" : "failure", netId,