Fix DCHECK error with GetDnsServers.

The LinkProperties.GetPrivateDnsServerName API method can return null,
which causes a DCHECK error for Android Pie and above  when we convert
from Java to c++.

Bug: 994459
Change-Id: I5a365df252254bc6f3e8cab4576be8ebce9a589b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1757352
Commit-Queue: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687633}

net/android/java/src/org/chromium/net/DnsStatus.java

@@ -25,7 +25,7 @@ public class DnsStatus {
             List<InetAddress> dnsServers, boolean privateDnsActive, String privateDnsServerName) {
         mDnsServers = dnsServers;
         mPrivateDnsActive = privateDnsActive;
-        mPrivateDnsServerName = privateDnsServerName;
+        mPrivateDnsServerName = (privateDnsServerName != null) ? privateDnsServerName : "";
     }
 
     @CalledByNative