Fix Error 500s loading repositories with no master branch

We removed the exception handling for Rugged errors in !16770, which
revealed that the licensee gem attempts to retrieve a license file
via Rugged in `refs/heads/master` by default. If that branch
did not exist, a Rugged::ReferenceError would be thrown.

There were two issues:

1. Not every project uses `master` as the default branch. This
change uses the head commit to identify the license.

2. Removing the exception handling caused repositories to fail
loading. We can safely catch and ignore any Rugged error because
this means we weren't able to load a license file.

Closes #43268

app/models/repository.rb

@@ -592,8 +592,14 @@ class Repository
   def license_key
     return unless exists?
-    Licensee.license(path).try(:key)
+    return unless head_commit
+    # 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)
+    rescue Rugged::Error
+    end
   end
   cache_method :license_key

spec/models/repository_spec.rb

@@ -873,6 +873,19 @@ describe Repository do
       expect(repository.license_key).to be_nil
     end
+    it 'returns nil when the commit SHA does not exist' do
+      allow(repository.head_commit).to receive(:sha).and_return('1' * 40)
+
+      expect(repository.license_key).to be_nil
+    end
+
+    it 'returns the license key even when master does not exist' do
+      repository.rm_branch(user, 'master')
+      project.change_head('test')
+
+      expect(repository.license_key).to eq('mit')
+    end
+
     it 'returns the license key' do
       repository.create_file(user, 'LICENSE',
         Licensee::License.new('mit').content,