YAML checker incorrectly rejects pipeline that omits `image_key_path` param

HTTomo allows the omission of the `image_key_path` parameter when the input data contains no image key (due to containing only projections, no darks or flats). However, the YAML checker incorrectly rejects such pipelines.

A workaround would be to provide `null` as the value of the `image_key_path` parameter in such cases, but it would be nicer to be able to omit the parameter entirely.