[xds] Fix test compilation (#33373)

Fix #33308
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test/core/xds/xds_client_test.cc

@@ -82,6 +82,7 @@ class XdsClientTest : public ::testing::Test {
    public:
     class FakeNode : public Node {
      public:
+      FakeNode() = default;
       const std::string& id() const override { return id_; }
       const std::string& cluster() const override { return cluster_; }
       const std::string& locality_region() const override {