Resolves mojohaus#850: Protecting the display-plugin-updates mojo aga…

…inst a NPE in case the effective version of a plugin is not specified

versions-maven-plugin/src/main/java/org/codehaus/mojo/versions/DisplayPluginUpdatesMojo.java

@@ -37,6 +37,7 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.Set;
 import java.util.Stack;
 import java.util.TreeMap;
@@ -52,6 +53,8 @@
 import org.apache.maven.model.BuildBase;
 import org.apache.maven.model.Model;
 import org.apache.maven.model.Plugin;
+import org.apache.maven.model.PluginConfiguration;
+import org.apache.maven.model.PluginManagement;
 import org.apache.maven.model.Prerequisites;
 import org.apache.maven.model.Profile;
 import org.apache.maven.model.ReportPlugin;
@@ -620,7 +623,7 @@ private static String pad(String start, int len, String... ends) {
         buf.append(start);
         int padding = len;
         for (String end : ends) {
-            padding -= end.length();
+            padding -= String.valueOf(end).length();
         }
         buf.append(' ');
         while (buf.length() < padding) {