RUBY-11500: added pseudo tag_start/tag_name for implicit tags

#RUBY-11500 fixed

platform/core-impl/src/com/intellij/lang/ASTFactory.java

@@ -92,6 +92,7 @@ public abstract class ASTFactory {
     return customLeaf != null ? customLeaf : DefaultFactoryHolder.DEFAULT.createLeaf(type, text);
   }
 
+  @Nullable
   private static ASTFactory factory(final IElementType type) {
     return LanguageASTFactory.INSTANCE.forLanguage(type.getLanguage());
   }