Relax block data contract

[Migaroez]

Description

#18275 is caused by

• A very strict data contract
• Data in the database that does not adhere to the contract

When the migration runs, part of the nested data fails to deserialize and to not make the entire migration fail, a message is logged and the data is cleared.
Since we currently have been unable to figure out how the malformed data has been entered trough the client and we think relaxing the data contract will not have any adverse effects, this pr does just that

Testing

I added a unit test that covers the change. You should be able to update the database manually to contain some problematic data. If not, feel free to reach out to obtain a database.

[AndyButland]

To test this I've:

• Taken a V13 installation with a rich text field.
• Manipulated the value in umbracoPropertyData to remove the blocks element from the stored JSON.
• Upgraded to 15 using the current state of v15/dev
• Found the markup removed and the following in the logs:

Could not parse rich text editor value, see exception for details.
System.Text.Json.JsonException: JSON deserialization for type 'Umbraco.Cms.Core.RichTextEditorValue' was missing required properties including: 'blocks'.

• Repeated using the code from this PR and can confirm the markup is displayed in the RTE as expected.
• When the content is saved the database is updated again with the empty blocks element.