Fix some specs, improve config

Allow empty string to unset nostr relay URL config

spec/services/nostr_manager/publish_zap_receipt_spec.rb

@@ -4,6 +4,10 @@ RSpec.describe NostrManager::PublishZapReceipt, type: :model do
   let(:user) { create :user, ln_account: "123456abcdef" }
   let(:zap) { create :zap, user: user }
 
+  before do
+    Setting.nostr_relay_url = ""
+  end
+
   describe "Default/delayed execution" do
     it "publishes zap receipts to all requested relays" do
       expect(NostrPublishEventJob).to receive(:perform_later)