fix: WP User core field support fixed in updateCustomer mutation

[kidunot89]

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What does this implement/fix? Explain your changes.

• updateCustomer mutation patched to support inputs without IDs for authenticated users.
• CustomerMutationsTest refactored heavily.

Does this close any currently open issues?

Fixes #784

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

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Any other comments?

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Where has this been tested?

• WooGraphQL Version: …
• WPGraphQL Version: …
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