let operator fix publications without tables

[FxKu]

We had the situation that a publication, slot and stream CRD were there but no publication tables. Not sure, how we ended up there. Maybe the stream operator created it after our operator did not block the slot and crd creation even when the table did not exist yet.

No publication tables leads to the following error in the operator:

could not sync publications in database "my_db": could not get current publications: error when processing row: sql: Scan error on column index 1, name "string_agg": converting NULL to string is unsupported

To support NULL values one can use go.sql's NullString implementation to catch this. But what to do next then? Set publications[pubName] = nil? This would make a later comparison between desired and current tables impossible.

Instead, this PR suggest to return an empty string. On sync the diff should be noticed and the operator would alter the publication and set a new table list.

[FxKu]

👍

[sdudoladov]

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