Merge pull request #778 from EOSIO/1.7.x/bin-extension-fix

release/1.7.x `binary_extension` and explicit default constructors fix

Author: arhag

libraries/eosiolib/core/eosio/binary_extension.hpp

@@ -103,12 +103,12 @@ namespace eosio {
          }
          constexpr T value_or()& {
             if (!_has_value)
-               return {};
+               return T();
             return _get();
          }
          constexpr T value_or()const& {
             if (!_has_value)
-               return {};
+               return T();
             return _get();
          }