Fix the default constructor of the optional class to avoid a spurious uninitialized value warning in older GCC versions (seen for ARM32 and GCC 8.3).

Use a union of a byte and the real storage, and init the char by default.

lib/include/srsran/adt/optional.h

@@ -24,7 +24,7 @@ class optional
 public:
   using value_type = T;
 
-  optional() : has_val_(false) {}
+  optional() : has_val_(false), empty() {}
   optional(const T& t) : has_val_(true) { storage.emplace(t); }
   optional(T&& t) : has_val_(true) { storage.emplace(std::move(t)); }
   optional(const optional<T>& other) : has_val_(other.has_value())
@@ -99,7 +99,10 @@ public:
 
 private:
   bool has_val_;
+  union {
+    char                    empty;
     detail::type_storage<T> storage;
+  };
 };
 
 template <typename T>