FIX: Absent scope in serializer will cause reports to fail

[nattsw]

There is a little bug now due to data-explorer. Below is the stack trace:

plugins/discourse-translator/plugin.rb:75:in `block (2 levels) in activate!'
lib/discourse_plugin_registry.rb:293:in `apply_modifier'
app/serializers/basic_topic_serializer.rb:9:in `fancy_title'
active_model_serializers (0.8.4) lib/action_controller/serialization.rb:50:in `block (2 levels) in <module:Serialization>'
...
plugins/discourse-data-explorer/app/controllers/discourse_data_explorer/query_controller.rb:191:in `block (2 levels) in run'

This PR makes us be a bit more resilient when there is no scope present in the serializer.