Sends json error when '_export_data_grouped' exists but not array.

This change retains PHP Warning message (though via JSON) to alert
developer where the problem exists.

Why? To ensure it does not silently pass through this code, which
would make debugging harder.

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

@@ -350,12 +350,6 @@ function wp_privacy_generate_personal_data_export_file( $request_id ) {
 		$email_address
 	);
 
-	// And now, all the Groups.
-	$groups = get_post_meta( $request_id, '_export_data_grouped', true );
-	if ( ! is_array( $groups ) ) {
-		$groups = array();
-	}
-
 	// First, build an "About" group on the fly for this report.
 	$about_group = array(
 		/* translators: Header for the About section in a personal data export. */
@@ -384,10 +378,24 @@ function wp_privacy_generate_personal_data_export_file( $request_id ) {
 		),
 	);
 
-	// Merge in the special about group.
-	$groups = array_merge( array( 'about' => $about_group ), $groups );
+	// And now, all the Groups.
+	$groups = get_post_meta( $request_id, '_export_data_grouped', true );
 
-	$groups_count = count( $groups );
+	// Merge in the special about group.
+	if ( is_array( $groups ) ) {
+		$groups       = array_merge( array( 'about' => $about_group ), $groups );
+		$groups_count = count( $groups );
+	} elseif ( '' === $groups || false === $groups ) {
+		$groups       = array( 'about' => $about_group );
+		$groups_count = 1;
+	} else {
+		wp_send_json_error(
+			sprintf(
+				'Warning: array_merge(): Expected parameter 2 to be an array, %1$s given.',
+				gettype( $groups )
+			)
+		);
+	}
 
 	// Convert the groups to JSON format.
 	$groups_json = wp_json_encode( $groups );