fix: preserve attribute values when extending complexType

[swozny13]

When a complexType with only attributes was extended by another complexType, the attributes were being rendered as "NotSet" instead of their actual values. This occurred because a placeholder element was unnecessarily created during type extension, causing the base type to be rendered twice - once with correct values and once with empty values that overwrote the correct ones.

Fixes #991

[swozny13]

Zeep version: 4.3.1

Example schema

<?xml version="1.0"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
        targetNamespace="http://example.com/test"
        xmlns:tns="http://example.com/test"
        elementFormDefault="unqualified"
        attributeFormDefault="unqualified">

  <complexType name="BaseType">
    <attribute name="attr1" type="integer" use="required"/>
    <attribute name="attr2" type="string" use="required"/>
  </complexType>

  <complexType name="ExtendedType">
    <complexContent>
      <extension base="tns:BaseType"/>
    </complexContent>
  </complexType>

  <element name="item" type="tns:ExtendedType"/>
</schema>

Reproduction

item = client.get_type('tns:ExtendedType')
obj = item(attr1=10, attr2="test")
print(etree.tostring(render(obj)))

Before fix

<item attr1="NotSet" attr2="NotSet"/>

After fix

<item attr1="10" attr2="test"/>