lutaml-model broken support of namespace-less child elements under a namespaced parent

The default Termium XML output looks like this:

```xml

...
```

Here, the `termium_extract` element has a namespace marked as `ns2`, but its children have no namespaces.

Technically, it should be parsed like this:

```ruby
module Termium
# For
class Extract < Lutaml::Model::Serializable
attribute :language, :string
attribute :extract_language, ExtractLanguage, collection: true
attribute :core, Core, collection: true

xml do
root "termium_extract"
namespace "http://termium.tpsgc-pwgsc.gc.ca/schemas/2012/06/Termium", "ns2"

map_attribute "language", to: :language, namespace: nil
map_element "extractLanguage", to: :extract_language, namespace: nil
map_element "core", to: :core, namespace: nil
end

end
end
```

However, this fails because it treats all its children with the same `ns2` namespace. Since the children elements have no namespace, they are all ignored.

Instead, this would work:

```ruby
module Termium
class Extract < Lutaml::Model::Serializable
attribute :language, :string
attribute :extract_language, ExtractLanguage, collection: true
attribute :core, Core, collection: true

xml do
root "termium_extract"

map_attribute "language", to: :language
map_element "extractLanguage", to: :extract_language
map_element "core", to: :core
end
end
end
```

Which is treating the element itself with no namespace, and the children have no namespace. This generates proper XML.

This is a bug in lutaml-model.

This can be found reproducible in the PR.