Disable 'use existing segmentation layer' when that layer doesn't exist

[philippotto]

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• open dashboard and use "create volume tracing" and "create hybrid tracing". verify that "use existing segmentation layer" is disabled if it doesn't exist

Superseded by #4939, but this PR is meant as a a hotfix.

[daniel-wer]

LGTM, unless @normanrz wanted the whole "Use ... Segmentation Layer" sub menu to be hidden if no segmentation layer exists (in that case a click on "Start Hybrid Annotation" could directly start a hybrid annotation with a new segmentation layer as there is no other option).

[normanrz]

Actually, I was hoping that the menu wouldn't be there at all and the annotation would be created right away.

[philippotto]

Yeah, #4939 will take care of this properly, but for this hotfix I decided to go with this minimal change, since the proper solution would be a bit more involved (and would cause several merge conflicts). Is this alright?

[normanrz]

Alright