bigquery connect doesn't use project_id if no credentials are passed

[toryhaavik]

ibis.bigquery.connect takes an arg project_id of type Optional[str]=None. It also takes an optional credentials arg. The way the code is written, if no credentials are passed, the user-passed project_id is thrown away in favor of the user's default project. I don't have a BQ account myself, but I think that some logic is needed to check if the user passes an explicit project_id and use that even if they pass None for credentials.