Fix nil error when comparing reports

ee/lib/gitlab/ci/reports/security/vulnerability_reports_comparer.rb

@@ -9,9 +9,9 @@ module Gitlab
 
           attr_reader :base_report, :head_report
 
-          def initialize(base_report = [], head_report = [])
-            @base_report = base_report
-            @head_report = head_report
+          def initialize(base_report, head_report)
+            @base_report = base_report || []
+            @head_report = head_report || []
           end
 
           def added

ee/spec/lib/gitlab/ci/reports/security/vulnerability_reports_comparer_spec.rb

@@ -52,12 +52,30 @@ describe Gitlab::Ci::Reports::Security::VulnerabilityReportsComparer do
   end
 
   describe 'with empty vulnerabilities' do
-    let(:comparer) { described_class.new }
-
     it 'returns empty array when reports are not present' do
-      expect(comparer.existing).to be_empty
-      expect(comparer.fixed).to be_empty
-      expect(comparer.added).to be_empty
+      comparer = described_class.new(nil, nil)
+
+      expect(comparer.existing).to eq([])
+      expect(comparer.fixed).to eq([])
+      expect(comparer.added).to eq([])
+    end
+
+    it 'returns added vulnerability when base is empty and head is not empty' do
+      vuln = build(:vulnerabilities_occurrence, report_type: :sast, identifiers: [identifier], location_fingerprint: '888')
+      comparer = described_class.new(nil, [vuln])
+
+      expect(comparer.existing).to eq([])
+      expect(comparer.fixed).to eq([])
+      expect(comparer.added).to eq([vuln])
+    end
+
+    it 'returns fixed vulnerability when head is empty and base is not empty' do
+      vuln = build(:vulnerabilities_occurrence, report_type: :sast, identifiers: [identifier], location_fingerprint: '888')
+      comparer = described_class.new([vuln], nil)
+
+      expect(comparer.existing).to eq([])
+      expect(comparer.fixed).to eq([vuln])
+      expect(comparer.added).to eq([])
     end
   end
 end