fix(graphql): handle nil value for locations in unified_trace

Author: codella

lib/datadog/tracing/contrib/graphql/unified_trace.rb

@@ -286,6 +286,10 @@ def add_query_error_events(span, errors)
           # is serialized as:
           #   ["3:10", "7:8"]
           def serialize_error_locations(locations)
+            # locations are only provided by the `graphql` library when the error can
+            # be associated to a particular point in the query.
+            return [] if locations.nil?
+
             locations.map do |location|
               "#{location["line"]}:#{location["column"]}"
             end