Fix node's search

[auroralimin]

Description
When editing a group's classification rule, it is possible to search for nodes to pin. However, when using a search substring that matches an already pinned node, the resulted list appears broken.

To Reproduce
Steps to reproduce the behavior:

1. Go to the Groups page
2. Click on 'classify' in one of the groups that have nodes
3. Click on 'rules' tab
4. Pin an node and commit changes
5. Search for the pinned node in an attempt to pin it again
6. See error

Expected behavior
When searching to pin a node that is already pinned, the node shouldn't appear in the nodes list and if the list is empty a "Nothing found" message should appear (as already happens in other situations)

Screenshots