Fix U4-4779: Media Picker not working as single picker

If you create a new Media Picker and leave "Multi Picker" unchecked, it still allows you to pick multiple items.

This is because `multiPicker` is `null` but the code only checks for a [specific condition](https://github.com/umbraco/Umbraco-CMS/blob/7.1.2/src/Umbraco.Web.UI.Client/src/views/propertyeditors/mediapicker/mediapicker.controller.js#L7).

This pull request updates the condition to be inline with the way it's done in the [dialog](https://github.com/umbraco/Umbraco-CMS/blob/7.1.2/src/Umbraco.Web.UI.Client/src/views/common/dialogs/mediapicker.controller.js#L10) and should fix the issue.