Meltano

Overriding a widcard with `null` yaml

```    metadata:
      "*":
        replication-method: INCREMENTAL
        replication-key: modified_date
      example_stream:
        key-properties: [key_id]
        replication-method: FULL_TABLE
        replication-key: null```
I remember having this question and issues around it with stream maps, and it was solved with a `_NULL_`  value or something in stream maps, but I'm not sure if we have a way to do this in metadata.

Right now what happens is the `replication-key` stays set to `modified_date` for the `example_stream` instead of being `NULL`

Which is an issue for us as the SDK still trys to pull information for State sign posts for FULL_TABLE syncs

Looks like setting `replication-key` to "" works (probably gets caught as non_truthy in the sdk so we're good :shrug:  )

For the specific error we were having of `modified_date` not existing in the schema a fix is to set `replication-key` to anything that Python truthyness recognizes as false

Thanks Derek. We should probably formalize it and add some tests to prevent regressions there.

<https://github.com/meltano/sdk/pull/3044>