Fix for null values

lib/blazer/data_source.rb

@@ -184,7 +184,7 @@ module Blazer
           columns = result.columns
           cast_method = Rails::VERSION::MAJOR < 5 ? :type_cast : :cast_value
           result.rows.each do |untyped_row|
-            rows << (result.column_types.empty? ? untyped_row : columns.each_with_index.map { |c, i| result.column_types[c].send(cast_method, untyped_row[i]) })
+            rows << (result.column_types.empty? ? untyped_row : columns.each_with_index.map { |c, i| untyped_row[i] ? result.column_types[c].send(cast_method, untyped_row[i]) : nil })
           end
         rescue ActiveRecord::StatementInvalid => e
           error = e.message.sub(/.+ERROR: /, "")