fix(ingestion): validate datasource in Tableau connector, before creating its upstream

[nandacamargo]

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)

Hi!

We are using Datahub Tableau connector and we are facing an issue when ingesting some Tableau data sources, as shown below:

File "/usr/local/lib/python3.8/site-packages/datahub/ingestion/source/tableau.py", line 324, in emit_custom_sql_datasources
    yield from self._create_lineage_from_csql_datasource(
File "/usr/local/lib/python3.8/site-packages/datahub/ingestion/source/tableau.py", line 397, in _create_lineage_from_csql_datasource
    self.platform, datasource.get("id", ""), self.config.env

AttributeError: 'NoneType' object has no attribute 'get'

This is happening because datasource is None and this PR adds a validation that fix this issue.

For the ingestion, we were testing with the following Datahub versions: 0.8.31 and 0.8.32.

Thank you!

[github-actions]

Unit Test Results (build & test)

  96 files  ±0    96 suites  ±0   23m 39s ⏱️ + 8m 24s
689 tests ±0  630 ✔️ ±0  59 💤 ±0  0 ❌ ±0 

Results for commit c99dba0. ± Comparison against base commit 5637e73.

♻️ This comment has been updated with latest results.

[github-actions]

Unit Test Results (metadata ingestion)

       5 files  ±    0         5 suites  ±0   54m 59s ⏱️ + 33m 20s
   429 tests +  33     424 ✔️ +  45    0 💤 ±0  0 ❌  - 12  5 🔥 ±0 
2 070 runs  +159  2 019 ✔️ +186  46 💤 +2  0 ❌  - 24  5 🔥  - 5 

For more details on these errors, see this check.

Results for commit c99dba0. ± Comparison against base commit 5637e73.

♻️ This comment has been updated with latest results.

[treff7es]

@mayurinehate please take a look

[mayurinehate]

Hey, thanks for raising the PR. I would like to suggest a change in the PR - to fix the cause of this error at its source.
I believe, simply adding a none check before adding datasource in csql's datasources at these lines https://github.com/datahub-project/datahub/blob/master/metadata-ingestion/src/datahub/ingestion/source/tableau_common.py#L478-L479 will fix the problem and existing changes in tableau.py can be reverted.

[shirshanka]

LGTM!