[MSHADE-428] Prevent null value in array of transformers (#229)

As Plexus will silently add it as explain it in issue.
This fix merely improves user experience, but not throwing cryptic or totally misleading errors.

---

https://issues.apache.org/jira/browse/MSHADE-428

src/main/java/org/apache/maven/plugins/shade/mojo/ShadeMojo.java

@@ -915,11 +915,16 @@ private List<Relocator> getRelocators() {
         return relocators;
     }
 
-    private List<ResourceTransformer> getResourceTransformers() {
+    private List<ResourceTransformer> getResourceTransformers() throws MojoExecutionException {
         if (transformers == null) {
             return Collections.emptyList();
         }
-
+        for (ResourceTransformer transformer : transformers) {
+            if (transformer == null) {
+                throw new MojoExecutionException(
+                        "Failed to create shaded artifact: parameter transformers contains null (double-check XML attribute)");
+            }
+        }
         return Arrays.asList(transformers);
     }