structuring plugin system

libmandel/src/CalcPlugin.cpp

@@ -22,9 +22,9 @@ CalcPlugin::~CalcPlugin(void)
 
 const std::vector<mnd::MandelGenerator*>& CalcPlugin::getGenerators(void)
 {
-    static std::vector<mnd::MandelGenerator*> empty = {};
+    static const std::vector<mnd::MandelGenerator*> empty = {};
     if (!isValid()) {
-        return {};
+        return empty;
     }
 
     using GeneratorGetter =
@@ -35,6 +35,6 @@ const std::vector<mnd::MandelGenerator*>& CalcPlugin::getGenerators(void)
         return gg();
     }
     else {
-        return nullptr;
+        return empty;
     }
 }