Derive macro panicks on anyType

[mtorromeo]

Hi,
while trying to derive structs from some xsd I experienced a panic in the derive macro with an unhelpful message:

error: proc-macro derive panicked
  --> src/main.rs:16:17
   |
16 | #[derive(Debug, XmlSchema)]
   |                 ^^^^^^^^^
   |
   = help: message: called `Option::unwrap()` on a `None` value

I tracked down the problem in the handling of anyType.

This is the minimal reproducible xsd that I could come up with:

<?xml version="1.0" encoding="UTF-8"?>

<schema xmlns="http://www.w3.org/2001/XMLSchema">

    <complexType name="mixedType">
        <complexContent>
            <restriction base="anyType"></restriction>
        </complexContent>
    </complexType>

</schema>

The panick is triggered by this unwrap: https://github.com/media-io/xml-schema/blob/a6a5b5800403d840e0d76eb6cd195c6854b1928d/xml_schema_derive/src/xsd/complex_content.rs#L19