prevent unintended m bp2build "" when running b

cl/2045666 allowed the --skip-soong-tests flag to pass through
invocations of the `b` command to soong_ui. But when the flag was
missing, an extra `""` argument was passed instead which caused soong_ui
to invoke a build of the droid target.

Test: m clean && b build '...'  # verify Soong does not run a build
Change-Id: I7b99437d225786eb1698a88d0c755ce897550e70