fix(bigquery): Do not generate null ordering on agg funcs

[VaggelisD]

Fixes #4170

BigQuery does not allow ASC NULLS LAST and DESC NULLS FIRST ordering in aggregate functions:

with colors as (SELECT 1 AS id, 'red' AS color) SELECT color, array_agg(id ORDER BY id ASC NULLS LAST) AS ids FROM colors GROUP BY 1

NULLS LAST not supported with ascending sort order in aggregate functions.

with colors as (SELECT 1 AS id, 'red' AS color) SELECT color, array_agg(id ORDER BY id DESC NULLS FIRST) AS ids FROM colors GROUP BY 1

NULLS FIRST not supported with descending sort order in aggregate functions.

Note that BigQuery is the only dialect with NULL_ORDERING_SUPPORTED = False so this PR uses this as a proxy to handle this case.

[georgesittas]

LGTM, one question to really understand this.