If a Parsons Table column has values like `[None, None, True, False]`,
the BigQuery connector will infer that the appropriate type for this
column is NoneType, which it will translate into a STRING type.

This change ensures that types returned by petl.typecheck() will
choose the first available type that isn't 'NoneType' if that is
available.

Source types ultimately come from `petl.typeset`, which calls
`type(v).__name__`. This call does not include source module, but only
the type name itself. e.g. `date` and not `datetime.date`

It looks like this line was accidentally commented out

Python datetime objects may represent timestamps or datetimes in
BigQuery, depending on whether they do or do not have a timezone
attached.

Always passing a schema to BigQuery is not necessary, and introduces
situations for provided schema to mismatch actual schema.

When table already exists in BigQuery, fetch the schema from BigQuery