Fix error when trying to authenticate user on repo with size check enabled

Bug was introduced in !6767

ee/spec/lib/gitlab/git_access_spec.rb

@@ -226,6 +226,12 @@ For more information: #{EE::Gitlab::GeoGitAccess::GEO_SERVER_DOCS_URL}"
         project.update_attribute(:repository_size_limit, 2.megabytes)
       end
 
+      context 'when trying to authenticate the user' do
+        it 'does not raise an error' do
+          expect { push_changes }.not_to raise_error
+        end
+      end
+
       context 'when pushing a new branch' do
         it 'accepts the push' do
           master_sha = project.commit('master').id

lib/gitlab/git/repository.rb

@@ -382,7 +382,7 @@ module Gitlab
       end
 
       def new_blobs(newrev)
-        return [] if newrev == ::Gitlab::Git::BLANK_SHA
+        return [] if newrev.blank? || newrev == ::Gitlab::Git::BLANK_SHA
 
         strong_memoize("new_blobs_#{newrev}") do
           wrapped_gitaly_errors do