Merge branch 'optional-plugin-dir' into 'development'

Don't run the plugin loader if the dir is missing

Closes #255

See merge request !154

core/src/main/java/nl/tudelft/ewi/auta/core/plugin/PluginLoader.java

@@ -22,6 +22,7 @@ import java.security.PrivilegedActionException;
 import java.security.PrivilegedExceptionAction;
 import java.util.ArrayList;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.stream.Collectors;
 
 /**
@@ -92,6 +93,11 @@ public class PluginLoader {
                 this.settings.get("plugins directory", String.class)
         );
 
+        if (Files.notExists(dir)) {
+            logger.warn("Plugins directory {} does not exist, skipping plugins", dir);
+            return Collections.emptyList();
+        }
+
         // Find all loadable jars in that directory
         final Collection<URL> urls;
         try (var files = Files.list(dir)) {