Modernize source code: last part (#6285)

* Modernize source code: last par

* Use empty when needed
* Use emplace_back instead of push_back when needed
* For range-based loops
* Initializers fixes
* constructors, destructors default
* c++ C stl includes

src/script/lua_api/l_metadata.h

@@ -31,7 +31,7 @@ class Metadata;
 class MetaDataRef : public ModApiBase
 {
 public:
-	virtual ~MetaDataRef() {}
+	virtual ~MetaDataRef() = default;
 
 protected:
 	static MetaDataRef *checkobject(lua_State *L, int narg);