message: optional: make coverity happy
diff: #include "envoy/common/exception.h"
  namespace Envoy {
  +
  /**
  * Contains an optional value. Like boost::optional and std::optional (not included in C++11).
  + * TODO: Replace with https://github.com/abseil/abseil-cpp/blob/master/absl/types/optional.h
  */
  template <typename T> class Optional {
  public:
  @@ -61,7 +63,8 @@ public:
  }
  private:
  - T value_;
  + T value_{};
  bool valid_{};
  };
  +
  } // namespace Envoy
  