blast: fix caching crash

Trying to cache a null buffer causes a crash. We shouldn't try to
cache them at all because they take up room in the cache and don't
have any peformance benefits. This patch adds support to just
send null buffers instead of caching them.

Bug: 129357029
Test: run Chrome, put it in the background and monitor the logs
Change-Id: I6dc026b5bc8b860a04bef1284c1a7adb4f1c96f3