test(config): add test for allow/2 nil pid no-op

Cover the allow(_owner_pid, nil) clause that silently succeeds when
Process.whereis returns nil for an unregistered name.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: solnic

test/sentry/test/config_isolation_test.exs

@@ -117,6 +117,10 @@ defmodule Sentry.Test.ConfigIsolationTest do
 
       Process.exit(shared_pid, :kill)
     end
+
+    test "allow/2 with nil pid is a no-op" do
+      assert :ok = Sentry.Test.Config.allow(self(), nil)
+    end
   end
 
   # Regression guard for I2: when two ancestors of the same descendant disagree