Add --empty-ninja-file for test usecases

In cases that we want to run blueprint-based builds in many
configurations to verify all of the logic works without errors, but
don't care about running the final ninja file, writing it out only
wastes time and disk space. So add a --empty-ninja-file option that
writes out an empty ninja file instead.

Our specific use case (Soong's build_test / multiproduct_kati) runs
Soong several hundred times for different configurations, and the ninja
files are around 1GB, which leads to several hundred gigabytes of disk
writes (and persistent use during incremental generation).

Change-Id: I0198dfb2f744ce22284c05d5214dac2ab5dc9700