Fix praefect test

Fixes an "undefined method `[]=`" error by adding an empty hash to the
array of node data if it doesn't include all 3 nodes.

qa/qa/specs/features/api/3_create/gitaly/distributed_reads_spec.rb

@@ -88,6 +88,7 @@ module QA
             read_from_project(project, number_of_reads)
 
             praefect_manager.query_read_distribution.each_with_index do |data, index|
+              diff_found[index] = {} unless diff_found[index]
               diff_found[index][:diff] = true if data[:value] > praefect_manager.value_for_node(pre_read_data, data[:node])
             end
             diff_found.all? { |node| node.key?(:diff) && node[:diff] }