Blank Group Filter results in no groups imported #30

Actual behaviour

When the Group Filter is left blank, the oauth2 plugin doesn't import any groups.

Expected behaviour

When the Group Filter is left blank, the oauth2 plugin imports all groups passed in the OAuth2 user info.

Steps to reproduce

Set the Group Filter to a blank string. Log in via OAuth with a user whom has groups assigned.

Configuration

• Plugin version: 1.0.2
• Kanboard version: 1.2.18
• Database type and version: MariaDB 10
• PHP version: 7.4
• OS: Debian 9
• Browser: Firefox