MTA-3184: Null xsi:type in variations causes an exception

dev/tests/functional/composer.json

 {
     "require": {
-        "magento/mtf": "1.0.0-rc50",
+        "magento/mtf": "1.0.0-rc51",
         "php": "~5.6.5|7.0.2|~7.0.6",
         "phpunit/phpunit": "~4.8.0|~5.5.0",
         "phpunit/phpunit-selenium": ">=1.2"