Some networks may have null ifaces, I guess?

Seems like a bug further upstream, but guard ourselves against it
for now.

Bug: 17033841
Change-Id: Id0e5895c3389b63d4e34b1fb064efef8b72cd609

core/java/android/net/LinkProperties.java

@@ -17,6 +17,7 @@
 package android.net;
 
 import android.annotation.NonNull;
+import android.annotation.Nullable;
 import android.net.ProxyInfo;
 import android.os.Parcelable;
 import android.os.Parcel;
@@ -124,7 +125,7 @@ public final class LinkProperties implements Parcelable {
      *
      * @return The interface name set for this link or {@code null}.
      */
-    public String getInterfaceName() {
+    public @Nullable String getInterfaceName() {
         return mIfaceName;
     }