Fix insert_overwrite incremental strategy in combination with required partition_filter fails when there are zero rows to insert

[tnk-ysk]

resolves #319
docs dbt-labs/docs.getdbt.com/#

Problem

If a model has incremental strategy and set to , an incremental run that tries to add zero (0) rows will fail and return from BigQuery.

Solution

Avoid require partition filter by adding partition is null or partition is not null to the merge condition.
Same solution as #986

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX