Call django_assets ./manage.py assets build if the app is present.

def build_assets():
    """Call django_assets ./manage.py assets build if the app is present."""
    cwd = os.getcwd()
    try:
        from webassets.script import GenericArgparseImplementation
        from django_assets.env import get_env
        prog = "%s assets" % os.path.basename(sys.argv[0])
        impl = GenericArgparseImplementation(
            env=get_env(), log=LOGGER, no_global_options=True, prog=prog)
        impl.run_with_argv(["build"])
    except ImportError:
        pass
    os.chdir(cwd)