[Backport] PRs 955 and 957 to 1.6.latest

[McKnight-42]

resolves #863
docs dbt-labs/docs.getdbt.com/#

Problem

A regression appears in the last release of dbt-bigquery. A user can previously add labels to a BigQuery job thanks to a macro returning a mapping. In the version 1.6.0, this was no longer possible even if the project is set to activate this feature.

Solution

#955
The proposed solution is to use, instead of self.profile.query_comment (which is the not-rendered Jinja macro), self.query_header.comment.query_comment (the JSON rendered query comments).

#957
to functionalize the small snippet and create a small change by making it to two nested if statements and moving the empty set of variable to top of function.

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