don't wrap null dates in moments

fixes CNVS-25102

test-plan:
 - go to create a new appointment group
 - enter just the date; should not get an error about start needing to
   be before end
 - enter just the date and start (no end); same
 - enter just the date and end (no start); same
 - enter a date and start and end, with start before end; no error
 - enter a date and start and end, with start after end; should get the
   error

Change-Id: I8ab3fddeef5d765f15c75e7f489201ea03e07714
Reviewed-on: https://gerrit.instructure.com/67325
Tested-by: Jenkins
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Gentry Beckmann <gbeckmann@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>

app/coffeescripts/util/fcUtil.coffee

@@ -8,6 +8,7 @@ define [
   # expects a date (unfudged), and returns a fullcalendar moment
   # (fudged) appropriate for passing to fullcalendar methods
   wrap = (date) ->
+    return null unless date
     $.fullCalendar.moment($.fudgeDateForProfileTimezone(date))
 
   # expects a fullcalendar moment (fudged) from a fullcalendar

spec/coffeescripts/calendar/TimeBlockRowSpec.coffee

@@ -92,6 +92,10 @@ define [
     me = new TimeBlockRow(@timeBlockList)
     ok me.validate()
 
+  test 'valid if incomplete', ->
+    me = new TimeBlockRow(@timeBlockList, start: @start, end: null)
+    ok me.validate()
+
   test 'getData', ->
     me = new TimeBlockRow(@timeBlockList, {@start, @end})
     me.validate()