Credit card start date field should default to empty

This is a usability problem reported by my client. When the credit card start date field is enabled, it defaults to January 1999. However, not all cards have a start date, and the checkout page itself even says "if present" next to the field. This can confuse customers as they cannot select a blank value here; some customers have called the store at this point as they are led to believe they must select a value here.

The attached patch fixes this by adding a blank row to the top of both the month and year start date dropdowns, and updates the start date validation function to succeed if both values are blank.