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)