fixup! chore: define an initial 'INIT group' model

Author: jgravois

benefits/enrollment_init/models.py

@@ -51,7 +51,9 @@ class Meta:
 
 
 class InitGroup(EnrollmentGroup):
-    group_id = models.IntegerField(default=None, blank=True, help_text="The ID of the INIT FareCategory for user enrollment.")
+    group_id = models.PositiveIntegerField(
+        default=None, blank=True, help_text="The ID of the INIT FareCategory for user enrollment."
+    )
 
     class Meta:
         verbose_name = "INIT group"