sql: allow cascading action when default is set implicitly to null

Add support for implicitly setting the default value of a column
to null, for cascading tables for both ON DELETE and ON UPDATE.

The problem was in updateRows within cascader.go when the column ids
were being updated for a table. If a user did not explicitly specify
NULL as the default value, the default expression was dereferenced,
which was null. Instead of deferencing said value, and setting it to the
column id, we now just set the column id to nil if the default
expression is also nil. This is the same behavior for the explicit case.

Resolves: cockroachdb#38975

Release note (sql change): Columns without an explicit default value now
support foreign keys with the SET DEFAULT action.

pkg/sql/create_table.go

@@ -593,14 +593,17 @@ func ResolveFK(
 		}
 	}
 
-	// Don't add a SET DEFAULT action on an index that has any column that does
-	// not have a DEFAULT expression.
+	// Don't add a SET DEFAULT action on an index that has any column that has
+	// a DEFAULT expression of NULL and a NOT NULL constraint.
 	if d.Actions.Delete == tree.SetDefault || d.Actions.Update == tree.SetDefault {
 		for _, sourceColumn := range srcCols {
-			if sourceColumn.DefaultExpr == nil {
+			// Having a default expression of NULL, and a constraint of NOT NULL is a
+			// contradiction and should never be allowed.
+			if sourceColumn.DefaultExpr == nil && !sourceColumn.Nullable {
 				col := qualifyFKColErrorWithDB(ctx, txn, tbl.TableDesc(), sourceColumn.Name)
 				return pgerror.Newf(pgcode.InvalidForeignKey,
-					"cannot add a SET DEFAULT cascading action on column %q which has no DEFAULT expression", col,
+					"cannot add a SET DEFAULT cascading action on column %q which has a "+
+						"NOT NULL constraint and a NULL default expression", col,
 				)
 			}
 		}