Insert and InsertUpdate loaders are inconsistent

[kdebisschop]

When 'columns' => [] in Insert loader, all columns are selected.

When 'columns' => [] in InsertUpdate loader, no columns are selected.

I think InsertUpdate is wrong and

if (is_array($this->columns) ?? [] !== $this->columns) {

should be

if (is_array($this->columns) && [] !== $this->columns) {

This bug will more or less automatically get resolved when we start using PHP 7.4 type hint for properties.

But I have a simple patch for the current code if we want to apply it...