Fixed return value for actionSaveSettings

[vnali]

Description
saving API Connection tab without filling inputs throws an error
craft\shopify\controllers\SettingsController::actionSaveSettings(): Return value must be of type yii\web\Response, null returned

this PR adds null to accepted return values.

[AugustMiller]

Hey, thanks for the report and patch! We had a couple more related items come up that we wanted to patch, but your changes have been incorporated into #40.

The only difference is that we've opted for craft\web\Response return types, and only made it nullable on the actionSaveSettings method, as actionIndex should always return a rendered template.