You can hit the "+"-sign in the staging branch section.

Then you can select from which branch it should fork the staging branch... 

Enter a name in "To" action and the staging branch will be created.

The staging branch is always a copy of the production db as far as i remember. the dev branches will be a demo database with the modules installed in the production branch...

So i think a new (or empty) db in staging section is not possible.

Create a new database without installing any Apps on another Server (maybe hosted locally). Take a backup and restore that backup to the staging branch. 

I did not see any possiblity , so we should have setup the local envirnoment of the vesrion we want to create database of and then created a raw DB there which can be downloaded and imported to SH staging branch.