MigrationLookup's skipOnEmpty() means you can't provide a default value if a lookup fails

I've made an attempt to implement the direction in #2. Attaching a patch for use with composer.

CC failure.

Tried to fix the custom command failure. Please review.

We are not adding any new exceptions-as-signals here. MigrateSkipProcessException is in the process of being deprecated in ✨ Allow process plugins to stop further processing on a pipeline RTBC . I confess it is odd and probably wrong that migration_lookup throws a skip process exception if the input is empty, yet will return a null when the lookup fails with a non-empty value, it does seem like we should treat both situations the same. Regardless, for BC and maintenance reasons we don't intend to make further changes to the migration lookup plugin. It is expected to be deprecated and replaced with a new one that handles this and other situations better in #3246666: Deprecate migrate_lookup and replace with cleaner process plugin → and that issue would be the place to make sure we include more consistent handling here. This will allow us to build a whole new plugin without worrying about backwards compatibility for these situations.

In case someone needs a default value when using the migration_lookup plugin: This is the work around that I am using:

  eid_lookup:
    plugin: migration_lookup
    source: user
    no_stub: true
    migration: user
  entity_id:
    plugin: default_value
    source: '@eid_lookup'
    default_value: 1