fix(ui): Correctly reset transforms

When removing a transform in the UI-based pipeline designer, we previously set the transform key to an empty string, which produced pipeline YAMLs like the following one:

spec:
  steps:
    fields:
      email:
        transform:
          key: "hash"
      name:
        transform:
          key: ""

These pipeline YAMLs failed when being evaluated with the Python runner since the runner could not find a transform with an empty key.

This commity changes this behavior and lets the pipeline designer correctly remove the transforms from the YAML, producing pipeline YAML that can be processed by the Python runner, e.g.:

spec:
  steps:
    fields:
      email:
        transform:
          key: "hash"

DataCater / datacater

fix(ui): Correctly reset transforms #53