blast: transaction ordering

This patch fixes two issues.

1) When a pending transactions is not found, don't try to use the iterator.
   This fixes a crash that happens when we are in a bad state.
2) When a transaction doesn't have any callbacks, don't try to send
   callbacks.

Bug: 130643588
Test: SurfaceFlinger_test
Change-Id: I6154c31dbf0b958683324c6b45e1a607691b84e8