Ignore nil to empty string changes

This fixes scenarios like (this one)[https://basecamp.com/1761222/projects/2937894-product-referral/todos/68257516-allow-override-of#comment_116998657] where the referral agreement override is submitted as an empty field and a dirty history record gets created as a change from nil to empty string.

![paula bishop - referrals - real hq contacts 2013-12-16 16-37-40](https://f.cloud.github.com/assets/372/1767146/f74bd5d2-674f-11e3-8536-dc017ad6146e.png)