Have generateKey() treat additional entropy as optional

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