lib/rb/lib/thrift/definition: Handle missing newly added symbol in the reflection

Author: AlexisMontagne

lib/rb/lib/thrift/definition.rb

@@ -15,10 +15,14 @@ def initialize(klass)
     end
 
     def structured_annotations
+      return [] unless @klass.const_defined? :STRUCTURED_ANNOTATIONS
+
       @klass::STRUCTURED_ANNOTATIONS
     end
 
     def legacy_annotations
+      return {} unless @klass.const_defined? :LEGACY_ANNOTATIONS
+
       @klass::LEGACY_ANNOTATIONS
     end