Fixed unitialized parameter in instances_path stylesheet, leading to
calls to add_instance with no data when passing datatypes insteatd of project
root. That bug was making exceptions (for exemple when editing type) when
having arrays of derivated types in project.

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                  <xsd:element name="Linux">

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                    <xsd:complexType>

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                      %(toolchain_gcc)s

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                    </xsd:complexType>

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                  </xsd:element>