Problem with custom siteUrl value

[daltonrooney]

In a58625d you added the ability to set a callback custom base URL for the callback, thanks! But I'm having a problem, my custom siteUrl value is not being used, and an extra slash is being added to my callback URL.

(It looks like the extra slash is always there, whether the custom siteUrl is set or not)

I believe the problem is here: https://github.com/enupal/socializer/blob/f3e8e2cfd6b34010cc9738a8a78719917785da8d/src/services/Settings.php#L65C28-L65C49

If UrlHelper::isProtocolRelativeUrl($settingsBaseUrl) returns false, $settingsBaseUrl is never applied.

I even tried setting my $siteUrl value as a relative protocol, and got an error "Hybridauth\Exception\InvalidArgumentException: A valid callback url is required."