Generate update set on conflict for upserts

[kaaveland]

When upserting to tables with only key columns, typo would generate do nothing as a conflict resolution. This has the disadvantage of making the query not execute the returning part, which meant that in cases where an upsert was executed targeting a single row, no rows would return and the generated code would cause an error.

As a workaround, we generate update set k = excluded.k for an arbitrary key column k instead. This causes the returning part of the query to run but shouldn't change the value of the row as typo sees it.

• Add failing test case for upserting on an existing row
• Make the test case work by generating update set instead of do nothing