## What changes were proposed in this pull request?

When fitting a PySpark Pipeline without the `stages` param set, a confusing NoneType error is raised as attempts to iterate over the pipeline stages.  A pipeline with no stages should act as an identity transform, however the `stages` param still needs to be set to an empty list.  This change improves the error output when the `stages` param is not set and adds a better description of what the API expects as input.  Also minor cleanup of related code.

## How was this patch tested?
Added new unit tests to verify an empty Pipeline acts as an identity transformer

Author: Bryan Cutler <cutlerb@gmail.com>

Closes #12790 from BryanCutler/pipeline-identity-SPARK-15018.