Fix Gitaly auto-detection caching

If `GitalyClient#can_use_disk?` returned `false`, it was never cached
properly and led to excessive number of Gitaly calls. Instead of using
`cached_value.present?`, we need to check `cached_value.nil?`.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64802

changelogs/unreleased/sh-fix-gitaly-server-info-cache.yml

+---
+title: Fix Gitaly auto-detection caching
+merge_request: 30954
+author:
+type: performance

lib/gitlab/gitaly_client.rb

@@ -392,7 +392,7 @@ module Gitlab
         @can_use_disk[storage]
       end
 
-      return cached_value if cached_value.present?
+      return cached_value unless cached_value.nil?
 
       gitaly_filesystem_id = filesystem_id(storage)
       direct_filesystem_id = filesystem_id_from_disk(storage)

spec/lib/gitlab/gitaly_client_spec.rb

@@ -119,6 +119,19 @@ describe Gitlab::GitalyClient do
     end
   end
 
+  describe '.can_use_disk?' do
+    it 'properly caches a false result' do
+      # spec_helper stubs this globally
+      allow(described_class).to receive(:can_use_disk?).and_call_original
+      expect(described_class).to receive(:filesystem_id).once
+      expect(described_class).to receive(:filesystem_id_from_disk).once
+
+      2.times do
+        described_class.can_use_disk?('unknown')
+      end
+    end
+  end
+
   describe '.connection_data' do
     it 'returns connection data' do
       address = 'tcp://localhost:9876'