Privacy: Ensure "Export Personal Data" does not generate invalid JSON.

Previously, when exporting personal data, if the JSON encoding of the data failed, the invalid JSON was still written to `export.json`.  This change captures the JSON encoding failure and adds a notice to the UI.  

Props hellofromTonya, jrf, SergeyBiryukov.
Fixes #52892.

git-svn-id: https://develop.svn.wordpress.org/trunk@50713 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/privacy-tools.php

@@ -413,6 +413,16 @@ function wp_privacy_generate_personal_data_export_file( $request_id ) {
 	// Convert the groups to JSON format.
 	$groups_json = wp_json_encode( $groups );
 
+	if ( false === $groups_json ) {
+		$error_message = sprintf(
+			/* translators: %s: Error message. */
+			__( 'Unable to encode the personal data for export. Error: %s' ),
+			json_last_error_msg()
+		);
+
+		wp_send_json_error( $error_message );
+	}
+
 	/*
 	 * Handle the JSON export.
 	 */