phpunit: Fix AvroFormatterTest failure on PHP 7

phpunit: Fix AvroFormatterTest failure on PHP 7

Fix the following error on PHP 7.

> MediaWiki\Logger\Monolog\AvroFormatterTest::testDoesSomethingWhenSchemaAvailable
> Only variables should be passed by reference
> includes/debug/logger/monolog/AvroFormatter.php:143

Per https://github.com/researchgate/avro-php/blob/1.8.0/lib/avro/schema.php#L311-L314
the default for &$schemata is null, which is filled with a plain AvroNamedSchemata
instance. So this parameter is obsolete.

Either it needs to be assigned here and then passed. But since we don't use
it anywhere and don't pass it any constructor arguments, the default
should suffice.

Bug: T75176
Bug: T141588
Change-Id: I144bed8a78eb267a97e41f379b89c5faaae30625