Merge 'use explicit null if it set instead of column default value' from Nikita Sivukhin

Before, tursodb always used default value even if NULL was explicitly
set by the user

Reviewed-by: Preston Thorpe <preston@turso.tech>

Closes #3422

core/vdbe/execute.rs

@@ -1712,7 +1712,9 @@ pub fn op_column(
 
                             match serial_type {
                                 // NULL
-                                0 => break 'ifnull,
+                                0 => {
+                                    state.registers[*dest] = Register::Value(Value::Null);
+                                }
                                 // I8
                                 1 => {
                                     state.registers[*dest] =

testing/insert.test

@@ -640,4 +640,13 @@ do_execsql_test_on_specific_db {:memory:} default-values-population {
     INSERT INTO t DEFAULT VALUES;
     SELECT * FROM t;
 } {1|666|
-2|666|}
+2|666|}
+
+do_execsql_test_on_specific_db {:memory:} set-explicit-null-default-value {
+    CREATE TABLE t (id INTEGER PRIMARY KEY, x DEFAULT 1);
+    INSERT INTO t(id, x) VALUES (1, 2);
+    SELECT * FROM t;
+    UPDATE t SET x = NULL WHERE id = 1;
+    SELECT * FROM t;
+} {1|2
+1|}