Translate ExecuteUpdate to null for a value set inside a JSON column #72

ExecuteUpdate that sets a value set inside a ToJson() column to null is refused (#57). It could be translated instead.

EF Core and Npgsql write the setter as jsonb_set("Doc", '{MaybeTags}', to_jsonb(NULL)). Measured against PostgreSQL 17:

SaveChanges writes a null value set as JSON null, and that reads back as null. So rewriting a setter whose value is the constant NULL (a null parameter is inlined as one) to 'null'::jsonb would write what SaveChanges writes. A value that may be null (CASE … THEN NULL ELSE @p END, a nullable column) would need COALESCE(to_jsonb(…), 'null'::jsonb), and its non-null branch still writes a JSON array, so it stays refused unless the non-null write is fixed as well.

Ranges and multiranges fail the same way, with Npgsql's 42804, so the same rewrite could cover them. That changes a query that threw into one that runs, so it's a 9.1.0 addition.

Open questions:

• Where to do the rewrite: in the processor decorator that runs ValueSetSqlGuard, or in a translator.
• Whether the non-null value set setter can be written as to_jsonb(CAST(@p AS text)), which gives the literal the document holds. That would need checking against the plugin's own literal (quoting, temporal precision) before anyone relies on it.