PluginConfig: Prevent NPE upon null extractGroupName value

Before this change, there was a NullPointerException upon running this
code when exactName ended up being null. GroupReference.extractGroupName
is a Nullable and can return a null value.

Change-Id: I6280cb1c98d54ac20e22fcc3725618c5db54df34
diff --git a/java/com/google/gerrit/server/config/PluginConfig.java b/java/com/google/gerrit/server/config/PluginConfig.java
index 2459d0b..2b363f1 100644
--- a/java/com/google/gerrit/server/config/PluginConfig.java
+++ b/java/com/google/gerrit/server/config/PluginConfig.java
@@ -156,7 +156,7 @@
 
   public Optional<GroupReference> getGroupReference(String name) {
     String exactName = GroupReference.extractGroupName(getString(name));
-    return groupReferences().values().stream().filter(g -> exactName.equals(g.getName())).findAny();
+    return groupReferences().values().stream().filter(g -> g.getName().equals(exactName)).findAny();
   }
 
   /** Mutable representation of {@link PluginConfig}. Used for updates. */