Work-around bug in Harmony CipherSpi

In Android pre-ICS the Harmony CipherSpi didn't accept "null" as a return
from engineUpdate. This change returns an empty byte array instead of
null which works around the problem.
[maven-release-plugin] prepare release sc-parent-1.47.0.3
[maven-release-plugin] prepare for next development iteration
Merge branch 'spongy-master' of github.com:rtyley/spongycastle into spongy-master