fix invalid json on an empty categories list (#2550)

Author: jcollins-g

lib/src/generator/dartdoc_generator_backend.dart

@@ -86,7 +86,7 @@ abstract class DartdocGeneratorBackend implements GeneratorBackend {
   @override
   void generateCategoryJson(
       FileWriter writer, List<Categorization> categories) {
-    var json = '';
+    var json = '[]';
     if (categories.isNotEmpty) {
       json = generator_util.generateCategoryJson(
           categories, options.prettyIndexJson);