Have generateKey() treat additional entropy as optional

KeyStore.generateKey() takes an entropy parameter. This is optional
and can be null. That is how KeyStore used to work but a recent
refactor made us always feed the entropy to keymaster, even if it
is empty (null or byte[0] on the Java side). This CL makes us ignore
such empty entropy again.

We only noticed this because a recently added GTS test that happens
to set the entropy to null is failing on some hardware (other
keymaster implementations silently ignore this invalid attempt to
set entropy).

Bug: 35156555
Test: gts-tradefed run gts --module GtsGmscoreHostTestCases
      --test com.google.android.gts.security.DeviceIdAttestationHostTest

Change-Id: Iadaf40e69350c17dd18e4dc2a1dab97fa911e1bf