Fix selection of datastore when uploading dataset

[philippotto]

URL of deployed dev instance (used for testing):

• https://___.webknossos.xyz

Steps to test:

• I hardcoded hasOnlyOneDatastoreOrNone to false so that the select UI becomes available when uploading a dataset
• without this PR, the UI immediately crashed when clicking on the select box
• with this PR, the dropdown does not crash anymore (upload works, too)
  I didn't test the select with multiple datastores (since I only have one set up currently), however, I think, the previous steps are already enough to show that the fix works.

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(Please delete unneeded items, merge only when none are left open)

• Updated (unreleased) changelog
• Ready for review

[MichaelBuessemeyer]

The fix work out for me 🟢 . I also only tested this locally with my local datastore ans setting hasOnlyOneDatastoreOrNone to false.

The code is also fine 👍 .

Let's 🚢 to 🔪 the 🐛