Make empty classifier null (not empty string) (#287)

And be consistent: as some legacy stuff in Maven and around (plugins) perform just nullcheck and not also empty string check, and get confused on empty string classifier. Plugins are (should) not be reused as dependencies, so despite the helper class is `public` I see no harm to alter it.

Supersedes #135 
Fixes #138

Author: cstamas

src/main/java/org/apache/maven/plugins/gpg/FilesCollector.java

@@ -33,6 +33,8 @@
 import org.codehaus.plexus.util.SelectorUtils;
 import org.eclipse.aether.artifact.Artifact;
 
+import static java.util.Objects.requireNonNull;
+
 /**
  * Collects project artifact, the POM, and attached artifacts to be signed.
  *
@@ -81,7 +83,7 @@ public List<Item> collect() throws MojoExecutionException, MojoFailureException
             File file = artifact.getFile();
 
             if (file != null && file.isFile()) {
-                items.add(new Item(file, artifact.getExtension()));
+                items.add(new Item(file, null, artifact.getExtension()));
             } else if (project.getAttachedArtifacts().isEmpty()) {
                 throw new MojoFailureException("The project artifact has not been assembled yet. "
                         + "Please do not invoke this goal before the lifecycle phase \"package\".");
@@ -103,7 +105,7 @@ public List<Item> collect() throws MojoExecutionException, MojoFailureException
             throw new MojoExecutionException("Error copying POM for signing.", e);
         }
 
-        items.add(new Item(pomToSign, "pom"));
+        items.add(new Item(pomToSign, null, "pom"));
 
         // ----------------------------------------------------------------------------
         // Attached artifacts
@@ -147,29 +149,32 @@ protected boolean isExcluded(Artifact artifact) {
 
     public static class Item {
         private final File file;
-
         private final String classifier;
-
         private final String extension;
 
         public Item(File file, String classifier, String extension) {
-            this.file = file;
-            this.classifier = classifier;
-            this.extension = extension;
-        }
-
-        public Item(File file, String extension) {
-            this(file, null, extension);
+            this.file = requireNonNull(file);
+            this.classifier = classifier == null || classifier.trim().isEmpty() ? null : classifier; // nullable
+            this.extension = requireNonNull(extension);
         }
 
+        /**
+         * The artifact backing file, never {@code null}.
+         */
         public File getFile() {
             return file;
         }
 
+        /**
+         * The classifier, if present, or {@code null}.
+         */
         public String getClassifier() {
             return classifier;
         }
 
+        /**
+         * The file extension (without leading period), never {@code null}.
+         */
         public String getExtension() {
             return extension;
         }