[Serializer] Fix XML attributes not added on empty node

[mtarld]

Q	A
Branch?	5.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Issues	Fix #52385
License	MIT

Add XML namespace attributes on node whether they have chlidren or not.

[mtarld]

fabbot errors are not related to that PR.

[nicolas-grekas]

@mtarld can you please have a look at #52401 and help us decide which approach we should merge? It looks like the test cases are the same, right?

[mtarld]

I think that there is an issue with #52401.

Indeed, the following XML:

<?xml version="1.0"?>
<response xmlns="http://www.w3.org/2005/Atom" xmlns:app="http://www.w3.org/2007/app" app:foo="bar">
</response>

will be decoded as:

[
    '@app:foo' => 'bar',
    '#' => '',
]

whereas it should in my opinion be decoded as:

[
    '@xmlns' => 'http://www.w3.org/2005/Atom',
    '@xmlns:app' => 'http://www.w3.org/2007/app',
    '@app:foo' => 'bar',
    '#' => '',
]

otherwise we lose the XML namespaces information.

But this is valid only if we do want to care about XML namespaces, of course.

[fabpot]

Thank you @mtarld.