C.20: define a ctor to not imply we're forbidding all ctors

closes #2212

CppCoreGuidelines.md

@@ -4878,6 +4878,7 @@ It's the simplest and gives the cleanest semantics.
 
     struct Named_map {
     public:
+        Named_map() : name("empty") {}
         // ... no default operations declared ...
     private:
         string name;