NPE when decoding null base 64 strings

**ByteArrayShufflePosition.fromBase64 is marked with a @Nullable argument, however it does not properly handle null inputs resulting in NPE.

This seems like an unintended change we picked up from the dependency: google-http-java-client/ switched from apache commons to guava (https://github.com/googleapis/google-http-java-client/commit/990c534f0e5103a142b0639c12c90cb990a00cfd#diff-97264fba16d690a26d63fbbc992af937) 

and decodeBase64 behaves differently in both cases. Former can handle null by returning null, latter will throw NPE.