Don't try to resolve empty impl rule class

Author: adangel

net.sourceforge.pmd.eclipse.plugin/src/main/java/net/sourceforge/pmd/eclipse/ui/preferences/panelmanagers/RulePanelManager.java

@@ -638,7 +638,9 @@ private boolean hasValidRuleType() {
         String newType = implementationClassField.getText();
         Class<?> newTypeClass = null;
         try {
-            newTypeClass = getClass().getClassLoader().loadClass(newType);
+            if (newType != null && !newType.isEmpty()) {
+                newTypeClass = getClass().getClassLoader().loadClass(newType);
+            }
         } catch (ClassNotFoundException e) {
             PMDPlugin.getDefault().logError("Couldn't find rule impl class " + newType, e);
         }