sql: incorrect "violates not-null constraint" error when UPSERTing a subset of columns

[mgartner]

When an UPSERT updates a subset of columns of an existing row, the values of non-updated columns are maintained. However, there is a bug were NOT NULL constraints are checked against the default column values, rather than the existing values. This causes UPSERT statements to incorrectly error with constraint violation errors. For example:

CREATE TABLE t (  
  id INT PRIMARY KEY DEFAULT unique_rowid(),  
  a INT NOT NULL,
  b INT
);  

INSERT INTO t (id, a, b) VALUES (1, 1, 1);  
-- INSERT 0 1

-- This should succeed. This UPSERT results in an update to the existing row,
-- and the existing value of "a" should be maintained. Therefore, the new row
-- does not violate the uniqueness check.
UPSERT INTO t (id, b) VALUES (1, 2);  
-- ERROR: null value in column "a" violates not-null constraint
-- SQLSTATE: 23502

This bug does not affect check constraints:

CREATE TABLE t (  
  id INT PRIMARY KEY DEFAULT unique_rowid(),  
  a INT DEFAULT -1 CHECK (a > 0),
  b INT
);  

INSERT INTO t (id, a, b) VALUES (1, 1, 1);  
-- INSERT 0 1

-- This succeeds as expected.
UPSERT INTO t (id, b) VALUES (1, 2);  
-- INSERT 0 1

SELECT * FROM t;

-- This fails as expected.
UPSERT INTO t (id, b) VALUES (2, 3);  
-- ERROR: failed to satisfy CHECK constraint (a > 0:::INT8)
-- SQLSTATE: 23514

This bug has existed since v20.1.0 or before.

Jira issue: CRDB-43475