Fix NativeThemeRebase null abort

ReleasePrimitiveArrayCritical will fail if the java object being
released is null. The array may be null if `style_count` is 0.
Do not call ReleasePrimitiveArrayCritical on the array if it is null.

Bug: 190927589
Test: forest
Change-Id: I432668f71137908838ebc3a47d834c1da3c67777