False conflicts when inserting in a not null field

[nuno-faria]

Describe the bug
MonetDB recently added the option to perform transactional non-conflicting inserts concurrently. However, if the field being inserted is defined as NOT NULL, one of the inserts fails. This also causes non-transactional inserts to conflict when the inserts contain multiple rows (e.g. INSERT INTO Test VALUES (1), (2), ...).

To Reproduce

• Create a test table with a NOT NULL field:

  CREATE TABLE Test (k int NOT NULL);

• Perform two concurrent transactions:

  -- T1
  BEGIN TRANSACTION;
  INSERT INTO Test VALUES (1);
  COMMIT;
  
  -- T2
  BEGIN TRANSACTION;
  INSERT INTO Test VALUES (2);
  COMMIT;
  
  -- last to commit is aborted

• If the table is instead created without the NOT NULL (CREATE TABLE Test (k int)), both inserts succeed.

Expected behavior
Both inserts to succeed.

Software versions

• MonetDB v11.43.6 Jan22
• Ubuntu 20.04 LTS
• Self-installed and compiled