Test `Table.test-upserts()`

[pydanny]

This PR does the following:

• Adds tests for fastlite's Table.upsert() method
• If an upsert's argument doesn't contain an input with fields/values, returns an empty dict
• If an upsert doesn't include a valid primary key, raises a MissingPrimaryKey error. This makes it easier to correct and move forward
• Works with both the current sqlite-minutils implementation and https://github.com/AnswerDotAI/sqlite-minutils/pull/42

[gitnotebooks[bot]]

Found 1 changed notebook. Review the changes at https://app.gitnotebooks.com/AnswerDotAI/fastlite/pull/44

[jph00]

Great!