Added default diagram generation error for empty diagrams (#246)

src/class_diagram/model/diagram.cc

@@ -249,6 +249,11 @@ void diagram::remove_redundant_dependencies()
     }
 }
 
+bool diagram::is_empty() const
+{
+    return element_view<class_>::is_empty() &&
+        element_view<enum_>::is_empty() && element_view<concept_>::is_empty();
+}
 } // namespace clanguml::class_diagram::model
 
 namespace clanguml::common::model {

src/class_diagram/model/diagram.h

@@ -249,6 +249,13 @@ public:
      */
     inja::json context() const override;
 
+    /**
+     * @brief Check whether the diagram is empty
+     *
+     * @return True, if diagram is empty
+     */
+    bool is_empty() const override;
+
 private:
     template <typename ElementT>
     bool add_with_namespace_path(std::unique_ptr<ElementT> &&e);