fix: renderpath for type-extensions

[m-hau]

The child_path variable can still be set from the attribute loop above, but then it has the wrong value. Without any attributes it is unset and causes an UnboundLocalError. Set it always to a proper value.

Fixes: 0c8a3fc ("Add support to pass base-types to type extensions")

[victorwon2001]

Confirmed the fix against 1b7072c: rendering a raw string through an attribute-free simpleContent extension raises UnboundLocalError, while 907f6c8 renders it correctly. The full suite passes locally (475 passed, 18 skipped), and Ruff is clean.

Could you add a regression test for that attribute-free path? A minimal schema with <xsd:simpleContent><xsd:extension base="xsd:string"/></xsd:simpleContent> and element.render(..., "hello") reproduces the failure on main. That would keep this one-line fix covered.

[m-hau]

I added a test like requested, making sure it fails without the fix and passes with the fix.

While looking at the other tests to get an idea how to add my own, I noticed one of the tests in that file called render_node(…) twice without any apparent necessity or effect. It is probably a simple copy&paste error, since some tests call that function before the expected constant and others right before the assert_nodes_equal. I took the liberty of adding a second commit removing the second redundant call.