fix(Stencila buildpack): Do not create `launch.toml` if no build entry
fix(Stencila buildpack): Do not create `launch.toml` if no build entry

stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila