Fix empty string start_date values on patient data criteria

### Story ###
- https://www.pivotaltracker.com/story/show/72535994
- http://jira.oncprojectracking.org/browse/BONNIE-6

### Notes ###
- this is an issue with the current [<code>production</code>](https://github.com/projecttacoma/bonnie/tree/production), but is not reproducible on the current [<code>master</code>](https://github.com/projecttacoma/bonnie/tree/master)
- replacing <code>.nil?</code> with <code>.blank?</code> resolves this issue on [<code>production</code>](https://github.com/projecttacoma/bonnie/tree/production), and should behave accordingly for [<code>master</code>](https://github.com/projecttacoma/bonnie/tree/master)
- this was tested using [CMS314v0](http://jira.oncprojectracking.org/browse/MUCD-63?jql=project%20%3D%20MUCD%20AND%20status%20%3D%20%221.%20Create%20%2F%20update%20eCQM%20bundle%22%20ORDER%20BY%20priority%20DESC) from a backup copy of production