[MNG-8180] Handle NPE due non-existent tags (#1641)

There was an NPE possibility when plugin.xml had no expected tags present.

Also: maven-compat has plugin.xml (!) w/o "name" tag, it NPEd and failed build. This was NOT picked up by CI as "rebuild itself" step does not install (just verify).

---

https://issues.apache.org/jira/browse/MNG-8180

maven-resolver-provider/src/main/java/org/apache/maven/repository/internal/PluginsMetadataGenerator.java

@@ -132,13 +132,14 @@ private PluginInfo extractPluginInfo(Artifact artifact) {
                             // - maven-plugin-api (for model)
                             // - Plexus Container (for model supporting classes and exceptions)
                             Xpp3Dom root = Xpp3DomBuilder.build(reader);
-                            String groupId = root.getChild("groupId").getValue();
-                            String artifactId = root.getChild("artifactId").getValue();
-                            String goalPrefix = root.getChild("goalPrefix").getValue();
-                            String name = root.getChild("name").getValue();
+                            String groupId = mayGetChild(root, "groupId");
+                            String artifactId = mayGetChild(root, "artifactId");
+                            String goalPrefix = mayGetChild(root, "goalPrefix");
+                            String name = mayGetChild(root, "name");
                             // sanity check: plugin descriptor extracted from artifact must have same GA
                             if (Objects.equals(artifact.getGroupId(), groupId)
                                     && Objects.equals(artifact.getArtifactId(), artifactId)) {
+                                // here groupId and artifactId cannot be null
                                 return new PluginInfo(groupId, artifactId, goalPrefix, name);
                             } else {
                                 throw new InvalidArtifactPluginMetadataException(
@@ -151,16 +152,25 @@ private PluginInfo extractPluginInfo(Artifact artifact) {
                             }
                         }
                     }
-                } catch (RuntimeException e) {
-                    throw e;
                 } catch (Exception e) {
+                    if (e instanceof InvalidArtifactPluginMetadataException) {
+                        throw (InvalidArtifactPluginMetadataException) e;
+                    }
                     // here we can have: IO. ZIP or Plexus Conf Ex: but we should not interfere with user intent
                 }
             }
         }
         return null;
     }
 
+    private static String mayGetChild(Xpp3Dom node, String child) {
+        Xpp3Dom c = node.getChild(child);
+        if (c != null) {
+            return c.getValue();
+        }
+        return null;
+    }
+
     public static final class InvalidArtifactPluginMetadataException extends IllegalArgumentException {
         InvalidArtifactPluginMetadataException(String s) {
             super(s);