Fix propagating EMPTY_NINJA_FILE from multiproduct_kati to minibp

I6dca478f356f56a8aee1e457d71439272351390b replaced calling
build/blueprint/bootstrap.bash with running minibp directly,
but didn't propagate the EMPTY_NINJA_FILE environment variable
to minibp.  Since everything that uses EMPTY_NINJA_FILE is
executed directly from soong_ui now, replace the EMPTY_NINJA_FILE
environment variable with a config bool and pass it to minibp.

Bug: 189148777
Test: build/soong/build_test.bash --dist --incremental --shard-count=300 --shard=1 && du out/multiproduct
Change-Id: I4d64275ce02c5d68948012f71ac4dc3795af9e85