Check that source and target branch exist

This ensures the return value is consistent if the source and/or
branch do not exist.

app/services/compare_service.rb

@@ -3,7 +3,7 @@
 require 'securerandom'
 
 # Compare 2 refs for one repo or between repositories
-# and return Gitlab::Git::Compare object that responds to commits and diffs
+# and return Compare object that responds to commits and diffs
 class CompareService
   attr_reader :start_project, :start_ref_name
 
@@ -15,7 +15,7 @@ class CompareService
   def execute(target_project, target_ref, base_sha: nil, straight: false)
     raw_compare = target_project.repository.compare_source_branch(target_ref, start_project.repository, start_ref_name, straight: straight)
 
-    return unless raw_compare
+    return unless raw_compare && raw_compare.base && raw_compare.head
 
     Compare.new(raw_compare,
                 target_project,

spec/controllers/projects/compare_controller_spec.rb

@@ -82,7 +82,7 @@ describe Projects::CompareController do
         show_request
 
         expect(response).to be_success
-        expect(assigns(:diffs).diff_files.to_a).to eq([])
+        expect(assigns(:diffs)).to eq([])
         expect(assigns(:commits)).to eq([])
       end
     end

spec/lib/gitlab/git/repository_spec.rb

@@ -1977,7 +1977,7 @@ describe Gitlab::Git::Repository, :seed_helper do
         expect(compare.commits.count).to be > 0
       end
 
-      it 'returns nil when source ref does not exist' do
+      it 'returns empty commits when source ref does not exist' do
         compare = repository.compare_source_branch('master', repository, 'non-existent-branch', straight: false)
 
         expect(compare.commits).to be_empty