Fix additional static analysis error

[tr33m4n]

Description

Fix an additional static analysis issue:

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  Line   vendor/ampersand/category-code/Model/CodedCategoryProductLink.php
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  50     Method Ampersand\CategoryCode\Model\CodedCategoryProductLink::getExtensionAttributes() should return
         Ampersand\CategoryCode\Api\Data\CodedCategoryProductLinkExtensionInterface|null but returns
         Magento\Framework\Api\ExtensionAttributesInterface.
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