Correct implicit assumption that group registration profiles will always have activity fields

Steps to reproduce:

• Create a profile "My Test Profile" with first and last name fields only. (Note: the important thing is that the profile does not contain any Activity fields.)
• Create a volunteer project "My Test Project" with the minimum required fields. Additionally, specify "My Test Profile" as the "Group Registration" profile.
• Visit /civicrm/vol/#/volunteer/opportunities and select "My Test Project," then click the Sign Up button in the "Commitments" box.
• Fill out the form, entering entering "1" in the Number of Additional Volunteers field
• Supply information for the "Additional Volunteer" and click submit.
• Witness the below error message (or find it in Drupal watchdog or elsewhere, depending on your environment and error reporting config):

Recoverable fatal error: Argument 1 passed to CRM_Volunteer_Form_VolunteerSignUp::processContactProfileData() must be of the type array, null given, called in /web/root/sites/default/ext/org.civicrm.volunteer/CRM/Volunteer/Form/VolunteerSignUp.php on line 618 and defined in CRM_Volunteer_Form_VolunteerSignUp->processContactProfileData() (line 575 of /web/root/sites/default/ext/org.civicrm.volunteer/CRM/Volunteer/Form/VolunteerSignUp.php).

The ramifications of this bug are that, until corrected, only contact fields may be used in group registration profiles.