fix: correctly fallback to server_url when no override is set

[Sheemap]

Hey yall! This should resolve #482

Tested this and its fully working!

I havent actually tested this yet, Ive been having some trouble getting a local environment up and running to test this. Its a super small fix though, and Im fairly confident this will work.

If someone could validate real quick that this works, or if anyone is available to help me debug my environment, that would be appreciated!

Cause

Whenever the server_url_override is stored, it is defaulted to empty string if a value is not set. This ?? check will only take the right side if the left is null though, empty string does not qualify. So inadvertently, the href property gets set to blank.

Proposed fix

Switching out the ?? operator for || should fix the issue. Empty string will not be truthy, and it will pass through to the server_url

[Sheemap]

I was able to figure out my local env issues and fully verify this fixes it! Would love a review whenever you have some time @MrDynamo