🐛 Source BigQuery: handle empty or null data set id

[prasrvenkat]

What

• Dataset is optional in BigQuery source
• Source BigQuery: Fix NPE when dataset_id isn't provided #6051 fixed it, except one flow.
• Looks like when creating a new source, UI does not send dataset_id if its not given.
  
• Since it's optional, it makes sense to properly check and only set if non-null in toDatabaseConfig

How

• Check optionally before adding it to config.

Recommended reading order

1. BigQuerySource.java
2. BigQuerySourceTest.java

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[marcosmarxm]

Awesome work @prasrvenkat !