fix: filter null files & added error notification

[headri]

When you share a collection with an element containing a NMR File and the detail level is < 3 (not allowed to read attachments):
There was an error from reading (null) files.

• Added check for null files.
• Added error message if all files are null because user is not allowed to read them.
  • But not if no files are found (e.g. no ids given)

Remaining issue

An empty NMR Editor is still opened. User may overlook error notification.
Editor is opened before files are fetched (async), therefore preventing it is not simple.
Maybe close it after getting files failed?

[github-actions]

LCOV of commit 6cc294c during Continuous Integration #4636

Summary coverage rate:
  lines......: 67.0% (15511 of 23139 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate: n/a

[github-actions]

LCOV of commit 2e3ddb7 during Continuous Integration #4642

Summary coverage rate:
  lines......: 67.0% (15495 of 23139 lines)
  functions..: no data found
  branches...: no data found

Files changed coverage rate: n/a