This avoids a fatal error on PHP 8 in `wp_privacy_generate_personal_data_export_file()` if the `_export_data_grouped` post meta exists but is not an array.
Additionally, refactor unit tests for the function to:
* Reduce redundant code
* Switch to data provider
* Test on the full HTML output instead of select pieces of the output
* Expand unhappy path coverage
Follow-up to [43012], [44786], [47146], [47278].
Props hellofromTonya, jrf, xknown.
See #51423.
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