Fix plugins' parent classloader to not include libraries, fixes #681 (#683)

diff --git a/daemon/src/main/java/org/mvndaemon/mvnd/plugin/CliMavenPluginManager.java b/daemon/src/main/java/org/mvndaemon/mvnd/plugin/CliMavenPluginManager.java
index 7cfc27b..9edb2b8 100644
--- a/daemon/src/main/java/org/mvndaemon/mvnd/plugin/CliMavenPluginManager.java
+++ b/daemon/src/main/java/org/mvndaemon/mvnd/plugin/CliMavenPluginManager.java
@@ -26,6 +26,8 @@
 import java.io.InputStream;
 import java.io.PrintStream;
 import java.io.Reader;
+import java.net.URL;
+import java.net.URLClassLoader;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -372,6 +374,9 @@
 
         pluginArtifacts = toMavenArtifacts(root, nlg);
 
+        if (parent == null) {
+            parent = new URLClassLoader(new URL[0]);
+        }
         pluginRealm = classRealmManager.createPluginRealm(plugin, parent, null, foreignImports,
                 toAetherArtifacts(pluginArtifacts));