ColumnValueProvider should support dealing with empty tables

[cptanalatriste]

Today, @cptanalatriste and I made changes / additions until we were able to successfully run `make-tables`, `make-generators`, `create-tables`, `create-generators` and `create-data` on the `cms_desynpuf_sample_53_large` schema:

- *could be in a separate PR* truncated String col types to account for a couple of `VARCHAR(1)`s
- made a custom config and generators file to account for the combined pk & fk of death.person_id
- *could be in a separate PR* returned None (IE no rows) from ColumnValueProvider rather than erroring if there are no rows in the table using `getattr(...,None)`

Originally posted by @Iain-S in #59 (comment)

This issue deals with the third item. We need ColumnValueProvider to return None when the target column is not populated, instead of crashing.