Add a flag to avoid to return a failure return value if no recipe is found

[traversaro]

The default behavior of rattler-build build is to fail if not recipe is found, and this make sense to early catch the case in which by error no recipe is passed. However, in some context this is the indeed behavior, for example if recipes are generated programmatically, and the programmatic generation produces intentionally no recipes to build.

This is the case in robostack, were we currently have dedicated checks in the CI that avoid to call rattler-build in case no recipe is generated (see https://github.com/RoboStack/ros-jazzy/blob/40eccf42ed9a4f8213b7f50dbe237e2e63f85c3a/.github/workflows/testpr.yml#L101-L108C1). It would be great to replace that logic with a --allow-empty-recipe-dir flag for rattler-build build or similar.