Show empty schemes as well

[matoubidou]

What does this PR do?

Empty schemes are shown in the ui as well.

Motivation

We are providing postgres databases to other developers inside the Daimler Group and currently recommending pgweb as a client. We got the feedback that it is a little bit confusing that you do not see empty schemes in the ui. Users might only learn about schemes when trying to create them via SQL. I was wondering if there is any special reason for this behaviour?

Best regards

_{Mathias Zeller [email protected] Daimler TSS GmbH (Imprint)}