Dataset is marked as being updated even when it is not being changed

[john-bodley]

Expected results

Only edits to the dataset metadata should trigger an update to the last updated event.

Actual results

When creating a new chart from dataset (not modify any columns or metrics) the dataset is listed as being updated.

Screenshots

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How to reproduce the bug

1. Create a new chart (no need to save it).
2. Simultaneously view the datasource in /tablemodelview/list/
3. Notice that you are listed as the person who last updated the datasource with the timestamp associated with (1).

Environment

• superset version: master
• python version: 3.8
• node.js version: 14

Checklist

Make sure to follow these steps before submitting your issue - thank you!

• I have checked the superset logs for python stacktraces and included it here as text if there are any.
• I have reproduced the issue with at least the latest released version of superset.
• I have checked the issue tracker for the same issue and I haven't found one similar.

Additional context

Add any other context about the problem here.

[john-bodley]

It seems if the table has a NULL extra column then upon creation of a chart a JSON encoded dictionary containing the health_check field is added resulting in an update to the record.