Handle missing .gitmodules when getting submodule urls

lib/gitlab/git/repository.rb

@@ -827,6 +827,8 @@ module Gitlab
         return unless commit_object && commit_object.type == :COMMIT
         gitmodules = gitaly_commit_client.tree_entry(ref, '.gitmodules', Gitlab::Git::Blob::MAX_DATA_DISPLAY_SIZE)
+        return unless gitmodules
+
         found_module = GitmodulesParser.new(gitmodules.data).parse[path]
         found_module && found_module['url']

spec/lib/gitlab/git/commit_spec.rb

@@ -310,8 +310,8 @@ describe Gitlab::Git::Commit, seed_helper: true do
             commits.map(&:id)
           end
-          it 'has 33 elements' do
-            expect(subject.size).to eq(33)
+          it 'has 34 elements' do
+            expect(subject.size).to eq(34)
           end
           it 'includes the expected commits' do

spec/lib/gitlab/git/repository_spec.rb

@@ -289,7 +289,13 @@ describe Gitlab::Git::Repository, seed_helper: true do
       it { expect(submodule_url('six')).to eq('git://github.com/randx/six.git') }
     end
-    context 'no submodules at commit' do
+    context 'no .gitmodules at commit' do
+      let(:ref) { '9596bc54a6f0c0c98248fe97077eb5ccf48a98d0' }
+
+      it { expect(submodule_url('six')).to eq(nil) }
+    end
+
+    context 'no gitlink entry' do
       let(:ref) { '6d39438' }
       it { expect(submodule_url('six')).to eq(nil) }
@@ -986,7 +992,7 @@ describe Gitlab::Git::Repository, seed_helper: true do
   describe '#branch_count' do
     it 'returns the number of branches' do
-      expect(repository.branch_count).to eq(9)
+      expect(repository.branch_count).to eq(10)
     end
   end

spec/support/gitlab-git-test.git/objects/95/96bc54a6f0c0c98248fe97077eb5ccf48a98d0

+x
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spec/support/gitlab-git-test.git/packed-refs

@@ -8,6 +8,7 @@
 46e1395e609395de004cacd4b142865ab0e52a29 refs/heads/gitattributes-updated
 4b4918a572fa86f9771e5ba40fbd48e1eb03e2c6 refs/heads/master
 5937ac0a7beb003549fc5fd26fc247adbce4a52e refs/heads/merge-test
+9596bc54a6f0c0c98248fe97077eb5ccf48a98d0 refs/heads/missing-gitmodules
 f4e6814c3e4e7a0de82a9e7cd20c626cc963a2f8 refs/tags/v1.0.0
 ^6f6d7e7ed97bb5f0054f2b1df789b39ca89b6ff9
 8a2a6eb295bb170b34c24c76c49ed0e9b2eaf34b refs/tags/v1.1.0

spec/support/seed_repo.rb

@@ -97,6 +97,7 @@ module SeedRepo
       gitattributes-updated
       master
       merge-test
+      missing-gitmodules
     ].freeze
     TAGS = %w[
       v1.0.0