Merge 'Fix: return NULL for rowid() when cursor's null flag is on' from Jussi Saurio

Fixes TPC-H query 13 from returning an incorrect result. In this
specific case, we were returning non-null `IdxRowid` values for the
right-hand side table even when there was no match with the left-hand
side table, meaning the join produced matches even in cases where there
shouldn't have been any.
Closes #2794

Closes #2795

core/storage/btree.rs

@@ -4321,6 +4321,9 @@ impl BTreeCursor {
                 return Ok(IOResult::Done(None));
             }
         }
+        if self.get_null_flag() {
+            return Ok(IOResult::Done(None));
+        }
         if self.has_record.get() {
             let page = self.stack.top();
             let page = page.get();

testing/join.test

@@ -302,3 +302,12 @@ do_execsql_test left-join-backwards-iteration {
 } {12|Alan|
 11|Travis|accessories
 10|Daniel|coat}
+
+# regression test for issue 2794: not nulling out rowid properly when left join does not match
+do_execsql_test_on_specific_db {:memory:} min-null-regression-test {
+    create table t (x integer primary key, y);
+    create table u (x integer primary key, y);
+    insert into t values (1,1),(2,2);
+    insert into u values (1,1),(3,3);
+    select count(u.x) from t left join u using(y);
+} {1}