Skip generation of empty or invalid relationships in PlantUML output

src/uml/class_diagram_model.h

@@ -17,6 +17,7 @@
  */
 #pragma once
 
+#include "util/error.h"
 #include "util/util.h"
 
 #include <clang-c/CXCompilationDatabase.h>
@@ -174,6 +175,13 @@ public:
 
     void add_relationship(class_relationship &&cr)
     {
+        if (cr.destination.empty() || type_aliases.count(cr.destination) == 0) {
+            LOG_WARN(
+                "Skipping relationship '{}' - {} - '{}' due to missing alias",
+                cr.destination, to_string(cr.type), usr);
+            return;
+        }
+
         auto it = std::find(relationships.begin(), relationships.end(), cr);
         if (it == relationships.end())
             relationships.emplace_back(std::move(cr));
@@ -281,7 +289,8 @@ struct diagram {
             }
         }
 
-        return full_name;
+        throw error::uml_alias_missing(
+            fmt::format("Missing alias for {}", full_name));
     }
 
     std::string usr_to_name(const std::vector<std::string> &using_namespaces,