"Open-Ended" opportunities should not have associated dates

Steps to reproduce:

1. Define a new opportunity with "Schedule Type" = "Open-Ended"
2. View "Available Volunteer Opportunities" public sign up form
3. Observe this newly defined opportunity listed here
4. Expect the "Time" column to be empty for this opportunity
5. Observe the "Time" column to have the following value: the date that the opportunity was added, and a time of 12:00 am

Further steps I took when troubleshooting

1. Find the row corresponding to the newly defined opportunity in civicrm_volunteer_need
2. Observe start_time to be set to the date described above
3. Set start_time to NULL instead
4. View "Available Volunteer Opportunities" again
5. Expect to see a row for this opportunity with no date
6. Observe that the row is not present

So, it seems to me that fixing this issue requires the following changes:

• Use NULL for start_date when creating a new opportunity with with "Schedule Type" = "Open-Ended"
• Display opportunities with no start date in "Available Volunteer Opportunities"

Tested on CiviVolunteer 2.2.1