Fix Error 500 when repository has no avatar

Closes #42249

lib/gitlab/git/blob.rb

@@ -132,6 +132,8 @@ module Gitlab
         end
         def find_by_gitaly(repository, sha, path, limit: MAX_DATA_DISPLAY_SIZE)
+          return unless path
+
           path = path.sub(/\A\/*/, '')
           path = '/' if path.empty?
           name = File.basename(path)
@@ -173,6 +175,8 @@ module Gitlab
         end
         def find_by_rugged(repository, sha, path, limit:)
+          return unless path
+
           rugged_commit = repository.lookup(sha)
           root_tree = rugged_commit.tree

spec/controllers/projects/avatars_controller_spec.rb

 require 'spec_helper'
 describe Projects::AvatarsController do
-  let(:project) { create(:project, avatar: fixture_file_upload(Rails.root + "spec/fixtures/dk.png", "image/png")) }
+  let(:project) { create(:project, :repository, avatar: fixture_file_upload(Rails.root + "spec/fixtures/dk.png", "image/png")) }
   let(:user)    { create(:user) }
   before do
@@ -10,6 +10,12 @@ describe Projects::AvatarsController do
     controller.instance_variable_set(:@project, project)
   end
+  it 'GET #show' do
+    get :show, namespace_id: project.namespace.id, project_id: project.id
+
+    expect(response).to have_gitlab_http_status(404)
+  end
+
   it 'removes avatar from DB by calling destroy' do
     delete :destroy, namespace_id: project.namespace.id, project_id: project.id
     expect(project.avatar.present?).to be_falsey

spec/lib/gitlab/git/blob_spec.rb

@@ -16,6 +16,18 @@ describe Gitlab::Git::Blob, seed_helper: true do
   end
   shared_examples 'finding blobs' do
+    context 'nil path' do
+      let(:blob) { Gitlab::Git::Blob.find(repository, SeedRepo::Commit::ID, nil) }
+
+      it { expect(blob).to eq(nil) }
+    end
+
+    context 'blank path' do
+      let(:blob) { Gitlab::Git::Blob.find(repository, SeedRepo::Commit::ID, '') }
+
+      it { expect(blob).to eq(nil) }
+    end
+
     context 'file in subdir' do
       let(:blob) { Gitlab::Git::Blob.find(repository, SeedRepo::Commit::ID, "files/ruby/popen.rb") }