1. Log in to Moodle as an admin and and go to Navigation > Site pages > Calendar.
2. Click the button 'Manage subscriptions'.
3. Try importing a calendar by filling in a name e.g. 'Site calendar', selecting to import from calendar file but do not upload a file in the filepicker, then click the Add button.
4. See how the event is added regardless of the fact no file was selected.

The form should check that a valid URL or an actual file was submitted. There is already a check to ensure it is the correct file type, so now it's simply an issue of checking whether it exists.