Fix fixed-rates calculation when missing cb fields

This if branch is very rare. And we do not set `metric_value` in it,
that means it takes metric_value from the previous loop or crashes.

Introduced in f3506b7.

app/models/chargeback_rate_detail.rb

@@ -24,10 +24,9 @@ class ChargebackRateDetail < ApplicationRecord
   def charge(relevant_fields, chargeback_fields_present, metric_rollup_records, hours_in_interval)
     result = {}
     if (relevant_fields & [metric_keys[0], cost_keys[0]]).present?
+      metric_value, cost = metric_and_cost_by(metric_rollup_records, hours_in_interval)
       if !chargeback_fields_present && fixed?
         cost = 0
-      else
-        metric_value, cost = metric_and_cost_by(metric_rollup_records, hours_in_interval)
       end
       metric_keys.each { |field| result[field] = metric_value }
       cost_keys.each   { |field| result[field] = cost }