Support boot classpath checksum without image

Add special case to ImageSpace::GetBootClassPathChecksums to handle
empty image space. In that case, we skip the image space part of the
checksum completely and print only the boot classpath "tail", which here
are all the boot classpath dex files.

This fixes a DCHECK failure in 692-vdex-inmem-loader.

Test: ./art/test/testrunner/run_build_test_target.py \
      art-interpreter-no-image
Bug: 72131483

Change-Id: I3deb2c4704a010214c2f4287cc3010cb5fb450dd