updates: Do not fail on updatable branches with no base

The golang-k8s-sigs-yaml update in apertis/v2021-updates broke the rendering for downstream instances since it does not have a base branch:

golang-k8s-sigs-yaml:
  git:
    path_with_namespace: pkg/golang-k8s-sigs-yaml
  updates:
  - branch:
      name: apertis/v2021-updates
      version: 1.2.0-2+apertis1
    upstream:
      component: development
      name: golang-k8s-sigs-yaml
      source: apertis/v2021-updates
      version: 1.2.0-3+apertis1

This is due to the fact that golang-k8s-sigs-yaml is a new dependency introduced by a package that got updated in apertis/v2021-updates and has not been folded into apertis/v2021 yet: no golang-k8s-sigs-yaml exists in apertis/v2021, and for this reason there's no base object.

In this case it is enough to trigger the pipeline directly on the apertis/v2021-updates branch.