Allow null subsample tables in peppy.Project

[nleroy917]

I was attempting to create a new peppy.Project object with the following config:

name: GSE96155
pep_version: 2.1.0
sample_table: GSE96155_annotation.csv
subsample_table: null
...

And peppy threw the following error: TypeError: Provided argument has to be a List[str] or a str, got 'NoneType'

Based on the discussion over at geofetch: pepkit/geofetch#80, peppy should allow null sub sample tables.

[nleroy917]

Low priority, but just saying this still exists as a bug.