web: Improve empty collaborators error message

Created by: philipp-spiess

Fixes https://github.com/sourcegraph/sourcegraph/issues/31011

As per https://github.com/sourcegraph/sourcegraph/issues/31011 this PR fixes the wording around various null states with the new invites feature.

More specifically we are now separating between two different null states:

• No collaborators found across any of the linked repos
• Custom search filter yields no results.

Below are two screenshots of the new null states.

Test plan

No collaborators found across any of the linked repos

Custom search filter yields no results