Fill unit only if we can determine unit from rates

otherwise

method ApplicationController.helpers.number_with_delimiter is called
 (thru  apply_format_function and format_currency_with_delimiter is called)
with parameters:

ApplicationController.helpers.number_to_currency('0.0', {:unit => nil} )

which is causing error.

app/models/miq_report/formatting.rb

@@ -69,7 +69,7 @@ def format(col, value, options = {})
     # Chargeback Reports: Add the selected currency in the assigned rate to options
     if Chargeback.db_is_chargeback?(db)
       @rates_cache ||= Chargeback::RatesCache.new
-      options[:unit] = @rates_cache.currency_for_report
+      options[:unit] = @rates_cache.currency_for_report if @rates_cache.currency_for_report
     end
 
     format.merge!(options) if format # Merge additional options that were passed in as overrides