Check for empty user-defined subsampling scheme

[huddlej]

Description of proposed changes

Adds a check for an empty user-defined subsampling scheme and provides a helpful error message when this happens to replace the unhelpful AttributeError message users would get before when the workflow tried to access a nonexistent method of a NoneType.

This error can happen when a user defines a custom subsampling scheme where the contents are not properly indented below the subsampling key. The resulting config is still a valid YAML file, so the emptiness of the subsampling scheme needs to be checked specifically. We could probably handle this better and more generically with a more stringent YAML schema.

Along the lines of that last point, I tried adding the following lines to the config schema to prevent an empty subsampling scheme:

  subsampling:
    type: object
    minProperties: 1

This change did not catch the issue with the user-defined configuration, however, because we call the validation function after we load the default config file and it is during this step that Snakemake throws an error (presumably when deep-merging the user-provided and default config files).

Next, I tried validating both before and after the configfile declaration in the Snakefile. This change catches the incorrect config file and prints the following error for the user:

WorkflowError in file /Users/jhuddlesfredhutch.org/projects/nextstrain/ncov/Snakefile, line 36:
Error validating config file.
ValidationError: None is not of type 'object'

Failed validating 'type' in schema['properties']['subsampling']:
    OrderedDict([('type', 'object'), ('minProperties', 1)])

On instance['subsampling']:
    None
  File "/Users/jhuddlesfredhutch.org/projects/nextstrain/ncov/Snakefile", line 36, in <module>

I like that this error gets generated from the schema without additional code added to the workflow itself. I wish the error from this validation step provided more details to the user about what specifically went wrong. The error I provide in the commit of this PR seems more helpful to the user, even though it requires a modification to the workflow

Testing

• Tested with a broken builds YAML where the subsampling scheme was not properly indented
• Tested with CI