Silence meaningless warnings from JarScanner

Don't log warnings when annotationName is null.

Change-Id: Ib7ef7490b1525a0d33ea09de6169ebde7759fb59
Signed-off-by: Dariusz Luksza <dariusz@luksza.org>

gerrit-server/src/main/java/com/google/gerrit/server/plugins/JarScanner.java

@@ -107,16 +107,16 @@ public class JarScanner implements PluginContentScanner {
         continue;
       }
 
-      if (def.isConcrete()) {
       if (!Strings.isNullOrEmpty(def.annotationName)) {
+        if (def.isConcrete()) {
             rawMap.put(def.annotationName, def);
-        }
         } else {
           PluginLoader.log.warn(String.format(
               "Plugin %s tries to @%s(\"%s\") abstract class %s", pluginName,
               def.annotationName, def.annotationValue, def.className));
         }
       }
+    }
 
     ImmutableMap.Builder<Class<? extends Annotation>, Iterable<ExtensionMetaData>> result =
         ImmutableMap.builder();