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

Description

Skips the plugin loader if the configured plugin directory does not exist. This is a common case if the core is being run for the first time. A warning is logged instead.

Additions

• Added a check to see if the directory exists. If it does not, a warning is logged and loading plugins is aborted.

Test and Review

Resolves #255 (closed)

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date

• All nullable parameters are marked as such

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)) {