Fix some specs, improve config

Allow empty string to unset nostr relay URL config

spec/requests/well_known_spec.rb

@@ -47,6 +47,10 @@ RSpec.describe "Well-known URLs", type: :request do
       end
 
       context "without relay configured" do
+        before do
+          Setting.nostr_relay_url = ""
+        end
+
         it "does not include a recommended relay" do
           get "/.well-known/nostr.json?name=bobdylan"
           res = JSON.parse(response.body)