Simplify license generator error handling

app/models/repository.rb

@@ -597,7 +597,7 @@ class Repository
     # The licensee gem creates a Rugged object from the path:
     # https://github.com/benbalter/licensee/blob/v8.7.0/lib/licensee/projects/git_project.rb
     begin
-      Licensee.project(path, revision: head_commit.sha).license.try(:key)
+      Licensee.license(path).try(:key)
     # Normally we would rescue Rugged::Error, but that is banned by lint-rugged
     # and we need to migrate this endpoint to Gitaly:
     # https://gitlab.com/gitlab-org/gitaly/issues/1026

spec/models/repository_spec.rb

@@ -879,11 +879,10 @@ describe Repository do
       expect(repository.license_key).to be_nil
     end
 
-    it 'returns the license key even when master does not exist' do
+    it 'returns nil when master does not exist' do
       repository.rm_branch(user, 'master')
-      project.change_head('test')
 
-      expect(repository.license_key).to eq('mit')
+      expect(repository.license_key).to be_nil
     end
 
     it 'returns the license key' do