/series/import/request/{id}: don't show price when it's empty

[php-coder]

It's possible that we save currency without price. It happens when a site parser is configured to use a static currency, price wasn't extracted (wrong selector, couldn't parse a string) and seller is known. In that case, we series_sales_import_parsed_data will have empty price field but non-empty currency field. This is how it looks in UI:

We should do the following:

1. don't save currency without price to database
2. migrate old data and NULL-ify records with currencies without prices