execute didn't work when SQL ends with a comment

Calling `execute` with SQL like the following didn't work:

```
INSERT INTO t (x, y, z) VALUES (7, 7, 7);
INSERT INTO t (x, y, z) VALUES (8, 8, 8);
-- this is a comment at the end
```

Executing that SQL results in a `Extralite::Error: not an error` error because `prepare_multi_stmt_impl` can set `stmt` to `NULL` when SQL ends with a comment. The documentation for `sqlite3_prepare_v2` states:

> If the input text contains no SQL (if the input is an empty string or a comment) then *ppStmt is set to NULL.

ext/extralite/database.c

@@ -294,6 +294,8 @@ static inline VALUE Database_perform_query(int argc, VALUE *argv, VALUE self, VA
   prepare_multi_stmt(DB_GVL_MODE(db), db->sqlite3_db, &stmt, sql);
   RB_GC_GUARD(sql);
 
+  if (stmt == NULL) return Qnil;
+
   bind_all_parameters(stmt, argc - 1, argv + 1);
   query_ctx ctx = QUERY_CTX(
     self, sql, db, stmt, Qnil, transform,
@@ -485,6 +487,10 @@ VALUE Database_query_single_array(int argc, VALUE *argv, VALUE self) {
  * specified using keyword arguments:
  *
  *     db.execute('update foo set x = :bar', bar: 42)
+ *
+ * @param sql [String] query SQL
+ * @param parameters [Array, Hash] parameters to run query with
+ * @return [Integer, nil] Total number of changes effected or `nil` if the query ends with a comment.
  */
 VALUE Database_execute(int argc, VALUE *argv, VALUE self) {
   return Database_perform_query(argc, argv, self, safe_query_changes, QUERY_HASH);

test/test_database.rb

@@ -948,7 +948,7 @@ def test_database_savepoint
 
       db.execute('insert into foo values (43)')
       assert_equal [42, 43], db.query_splat('select x from foo')
-      
+
       db.savepoint(:b)
 
       db.execute('insert into foo values (44)')
@@ -1046,6 +1046,48 @@ def test_wal_checkpoint
     assert File.size(wal_fn) == 0
     assert_equal [0, 0], r
   end
+
+  def test_execute_with_comments
+    result = @db.execute(<<~SQL)
+      -- this is a comment
+    SQL
+    assert_nil result
+
+    result = @db.execute(<<~SQL)
+      DELETE FROM t;
+      INSERT INTO t (x, y, z) VALUES (1, 1, 1);
+      INSERT INTO t (x, y, z) VALUES (2, 2, 2);
+    SQL
+    assert_equal 1, result
+    assert_equal [1, 2], @db.query_splat('SELECT x FROM t ORDER BY x')
+
+    result = @db.execute(<<~SQL)
+      -- this is a comment at the beginning
+      DELETE FROM t;
+      INSERT INTO t (x, y, z) VALUES (3, 3, 3);
+      INSERT INTO t (x, y, z) VALUES (4, 4, 4);
+    SQL
+    assert_equal 1, result
+    assert_equal [3, 4], @db.query_splat('SELECT x FROM t ORDER BY x')
+
+    result = @db.execute(<<~SQL)
+      DELETE FROM t;
+      INSERT INTO t (x, y, z) VALUES (5, 5, 5);
+      -- this is a comment in the middle
+      INSERT INTO t (x, y, z) VALUES (6, 6, 6);
+    SQL
+    assert_equal 1, result
+    assert_equal [5, 6], @db.query_splat('SELECT x FROM t ORDER BY x')
+
+    result = @db.execute(<<~SQL)
+      DELETE FROM t;
+      INSERT INTO t (x, y, z) VALUES (7, 7, 7);
+      INSERT INTO t (x, y, z) VALUES (8, 8, 8);
+      -- this is a comment at the end
+    SQL
+    assert_nil result
+    assert_equal [7, 8], @db.query_splat('SELECT x FROM t ORDER BY x')
+  end
 end
 
 class ScenarioTest < Minitest::Test
@@ -1438,7 +1480,7 @@ def test_progress_handler_reset
 
   def test_progress_handler_invalid_arg
     db = Extralite::Database.new(':memory:')
-    
+
     assert_raises(TypeError) { db.on_progress(period: :foo) }
     assert_raises(TypeError) { db.on_progress(tick: :foo) }
     assert_raises(ArgumentError) { db.on_progress(mode: :foo) }
@@ -1635,7 +1677,7 @@ def test_global_progress_handler
 class RactorTest < Minitest::Test
   def test_ractor_simple
     skip if SKIP_RACTOR_TESTS
-    
+
     fn = Tempfile.new('extralite_test_database_in_ractor').path
 
     r = Ractor.new do