Fix LFS push mirroring for mirrors set up a long time ago

We discovered this on our own gitlab-foss -> github mirror. Where the
credentials were generated a long time ago, they don't contain an
`auth_method`. That means the method is `password`. It would be nicer
to keep the details of that within the `RemoteMirror` class, but that
would ban direct access to the `credentials` hash.

Changelog: fixed

changelogs/unreleased/328674-fix-lfs-push-for-auth-method-password.yml

+---
+title: Fix LFS push mirroring for mirrors set up a long time ago
+merge_request: 59972
+author:
+type: fixed

lib/gitlab/lfs/client.rb

@@ -97,7 +97,10 @@ module Gitlab
       end
 
       def basic_auth
-        return unless credentials[:auth_method] == "password"
+        # Some legacy credentials have a nil auth_method, which means password
+        # https://gitlab.com/gitlab-org/gitlab/-/issues/328674
+        return unless credentials.fetch(:auth_method, 'password') == 'password'
+        return if credentials.empty?
 
         { username: credentials[:user], password: credentials[:password] }
       end

spec/lib/gitlab/lfs/client_spec.rb

@@ -6,7 +6,7 @@ RSpec.describe Gitlab::Lfs::Client do
   let(:base_url) { "https://example.com" }
   let(:username) { 'user' }
   let(:password) { 'password' }
-  let(:credentials) { { user: username, password: password, auth_method: 'password' } }
+  let(:credentials) { { user: username, password: password } }
   let(:git_lfs_content_type) { 'application/vnd.git-lfs+json' }
   let(:git_lfs_user_agent) { "GitLab #{Gitlab::VERSION} LFS client" }