ProductAttribute null key error fixed.

[kidunot89]

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What does this implement/fix? Explain your changes.

• Fixes null key error being through with attributeId is set to 0.
• Adds a cacheable global ID as id. Just like with the VariationAttribute id field is not design for retrieval purposes although it can be use that way. It's true purpose is for caching.

Does this close any currently open issues?

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Any relevant logs, error output, GraphiQL screenshots, etc?

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Any other comments?

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Where has this been tested?

Operating System: Ubuntu 18.04

WordPress Version: 5.2