Prunes whole Geo event when there's only a primary

When there is only a primary node configured, but no secondary nodes,
prune the complete Geo event log.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9712

ee/app/workers/geo/prune_event_log_worker.rb

@@ -13,7 +13,7 @@ module Geo
       return if Gitlab::Database.read_only?
       return unless Gitlab::Database.healthy?
 
-      unless ::GeoNode.any?
+      unless ::GeoNode.secondary_nodes.any?
         Geo::PruneEventLogService.new(:all).execute
         return
       end

ee/changelogs/unreleased/tc-geo-fix-prune-worker-nil.yml

+---
+title: Prunes whole Geo event when there's only a primary
+merge_request: 9630
+author:
+type: fixed

ee/spec/workers/geo/prune_event_log_worker_spec.rb

@@ -59,10 +59,9 @@ describe Geo::PruneEventLogWorker, :geo do
         worker.perform
       end
 
-      context 'no Geo nodes' do
+      context 'no Geo secondary nodes' do
         before do
           secondary.destroy
-          primary.destroy
         end
 
         it 'deletes everything from the Geo event log' do
@@ -72,6 +71,20 @@ describe Geo::PruneEventLogWorker, :geo do
 
           worker.perform
         end
+
+        context 'no Geo primary node' do
+          before do
+            primary.destroy
+          end
+
+          it 'deletes everything from the Geo event log' do
+            create_list(:geo_event_log, 2)
+
+            expect(Geo::PruneEventLogService).to receive(:new).with(:all).and_call_original
+
+            worker.perform
+          end
+        end
       end
 
       context 'multiple secondary nodes' do