Drop JSON null values on streams

[turb]

When sending data with loadJson, empty Option properties are sent as JSON null. While BigQuery then stores it as a column null for almost all column types, it is not the case with JSON column types.

For example:

• load with None => stored as JSON null (not column null)
• pull => returns Some(null)

The change is simple: just remove the null JSON properties so that BigQuery always stores a DB null, even with the JSON column type.

[turb]

Hello @mariuene @TSommerdal, do you think it would be possible to review / merge / release this?

Many thanks!