Return 0 early if there are no events

Add specs for the zero check

lib/gitlab/usage_data_counters/hll_redis_counter.rb

@@ -151,6 +151,9 @@ module Gitlab
           aggregation = events.first[:aggregation]
 
           keys = keys_for_aggregation(aggregation, events: events, start_date: start_date, end_date: end_date, context: context)
+
+          return FALLBACK unless keys.any?
+
           redis_usage_data { Gitlab::Redis::HLL.count(keys: keys) }
         end
 

spec/lib/gitlab/usage_data_counters/hll_redis_counter_spec.rb

@@ -307,6 +307,11 @@ RSpec.describe Gitlab::UsageDataCounters::HLLRedisCounter, :clean_gitlab_redis_s
         described_class.track_event(daily_event, values: entity4, time: 29.days.ago)
       end
 
+      it 'returns 0 if there are no keys for the given events' do
+        expect(Gitlab::Redis::HLL).not_to receive(:count)
+        expect(described_class.unique_events(event_names: [weekly_event], start_date: Date.current, end_date: 4.weeks.ago)).to eq(-1)
+      end
+
       it 'raise error if metrics are not in the same slot' do
         expect do
           described_class.unique_events(event_names: [compliance_slot_event, analytics_slot_event], start_date: 4.weeks.ago, end_date: Date.current)
@@ -526,6 +531,11 @@ RSpec.describe Gitlab::UsageDataCounters::HLLRedisCounter, :clean_gitlab_redis_s
       expect { described_class.calculate_events_union(**time_range.merge(event_names: %w[event1_slot event4])) }.to raise_error described_class::SlotMismatch
       expect { described_class.calculate_events_union(**time_range.merge(event_names: %w[event5_slot event3_slot])) }.to raise_error described_class::AggregationMismatch
     end
+
+    it 'returns 0 if there are no keys for given events' do
+      expect(Gitlab::Redis::HLL).not_to receive(:count)
+      expect(described_class.calculate_events_union(event_names: %w[event1_slot event2_slot event3_slot], start_date: Date.current, end_date: 4.weeks.ago)).to eq(-1)
+    end
   end
 
   describe '.weekly_time_range' do