StaticIpConfiguration: Couple of minor fixes

a) Ensure the Builder.build() does not crash if setDnsServers() is not
invoked.
b) Add a checkNotNull on setDnsServers() since it is marked @NonNull.

Bug: 144487020
Test: Compiles
Change-Id: I2b9f990efa0583c0f067c25e23d8c944ffbdf6a5
(cherry-picked from d0348339c50eeae0fe137c62476f6a01542f4163)

core/java/android/net/StaticIpConfiguration.java

@@ -25,6 +25,8 @@ import android.net.shared.InetAddressUtils;
 import android.os.Parcel;
 import android.os.Parcelable;
 
+import com.android.internal.util.Preconditions;
+
 import java.net.InetAddress;
 import java.util.ArrayList;
 import java.util.List;
@@ -152,6 +154,7 @@ public final class StaticIpConfiguration implements Parcelable {
          * @return The {@link Builder} for chaining.
          */
         public @NonNull Builder setDnsServers(@NonNull Iterable<InetAddress> dnsServers) {
+            Preconditions.checkNotNull(dnsServers);
             mDnsServers = dnsServers;
             return this;
         }
@@ -175,9 +178,11 @@ public final class StaticIpConfiguration implements Parcelable {
             final StaticIpConfiguration config = new StaticIpConfiguration();
             config.ipAddress = mIpAddress;
             config.gateway = mGateway;
+            if (mDnsServers != null) {
                 for (InetAddress server : mDnsServers) {
                     config.dnsServers.add(server);
                 }
+            }
             config.domains = mDomains;
             return config;
         }