Do not return empty issues if negated assignee does not exist

app/finders/issuable_finder.rb

@@ -395,8 +395,6 @@ class IssuableFinder
     # We want CE users to be able to say "Issues not assigned to either PersonA nor PersonB"
     if not_params.assignees.present?
       items.not_assigned_to(not_params.assignees)
-    elsif not_params.assignee_id? || not_params.assignee_username? # assignee not found
-      items.none
     else
       items
     end

spec/finders/issues_finder_spec.rb

@@ -49,6 +49,13 @@ RSpec.describe IssuesFinder do
           let(:expected_issuables) { [issue3, issue4] }
         end
 
+        context 'when assignee_id does not exist' do
+          it_behaves_like 'assignee NOT ID filter' do
+            let(:params) { { not: { assignee_id: -100 } } }
+            let(:expected_issuables) { [issue1, issue2, issue3, issue4, issue5] }
+          end
+        end
+
         context 'filter by username' do
           let_it_be(:user3) { create(:user) }
 
@@ -76,6 +83,17 @@ RSpec.describe IssuesFinder do
             let(:params) { { not: { assignee_username: [user.username, user2.username] } } }
             let(:expected_issuables) { [issue3, issue4] }
           end
+
+          context 'when assignee_username does not exist' do
+            it_behaves_like 'assignee NOT username filter' do
+              before do
+                issue2.assignees = [user2]
+              end
+
+              let(:params) { { not: { assignee_username: 'non_existent_username' } } }
+              let(:expected_issuables) { [issue1, issue2, issue3, issue4, issue5] }
+            end
+          end
         end
 
         it_behaves_like 'no assignee filter' do