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

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

@@ -268,7 +268,7 @@ def add_query_error_events(span, errors)
                   @type_key => parsed_error.type,
                   @stacktrace_key => parsed_error.backtrace,
                   @message_key => graphql_error['message'],
-                  @locations_key => serialize_error_locations(graphql_error['locations']),
+                  @locations_key => self.class.serialize_error_locations(graphql_error['locations']),
                   @path_key => graphql_error['path'],
                 )
               )
@@ -285,7 +285,11 @@ def add_query_error_events(span, errors)
           #   ]
           # is serialized as:
           #   ["3:10", "7:8"]
-          def serialize_error_locations(locations)
+          def self.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

sig/datadog/tracing/contrib/graphql/unified_trace.rbs

@@ -99,7 +99,7 @@ module Datadog
 
           def add_query_error_events: (SpanOperation span, Array[GraphQL::Error] errors) -> void
 
-          def serialize_error_locations: (Array[Hash[String, Integer]] locations) -> Array[String]
+          def self.serialize_error_locations: (Array[Hash[String, Integer]]? locations) -> Array[String]
         end
       end
     end

spec/datadog/tracing/contrib/graphql/unified_trace_spec.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require 'datadog/tracing/contrib/support/spec_helper'
+require 'datadog/tracing/contrib/graphql/unified_trace'
+
+RSpec.describe Datadog::Tracing::Contrib::GraphQL::UnifiedTrace do
+  describe '.serialize_error_locations' do
+    subject(:result) { described_class.serialize_error_locations(locations) }
+
+    context 'when locations is nil' do
+      let(:locations) { nil }
+
+      it 'returns an empty array' do
+        expect(result).to eq([])
+      end
+    end
+
+    context 'when locations is an array' do
+      let(:locations) do
+        [
+          { 'line' => 3, 'column' => 10 },
+          { 'line' => 7, 'column' => 8 }
+        ]
+      end
+
+      it 'maps locations to formatted strings' do
+        expect(result).to eq(['3:10', '7:8'])
+      end
+    end
+
+    context 'when locations is an empty array' do
+      let(:locations) { [] }
+
+      it 'returns an empty array' do
+        expect(result).to eq([])
+      end
+    end
+  end
+end
+