Handle the case where histogram score can be null

This will leave out the student that never attempt the question and
fixes the error with histogram rendering.

lms/static/coffee/spec/histogram_spec.coffee

@@ -11,7 +11,7 @@ describe 'Histogram', ->
 
   describe 'calculate', ->
     beforeEach ->
-      @histogram = new Histogram(1, [[1, 1], [2, 2], [3, 3]])
+      @histogram = new Histogram(1, [[null, 1], [1, 1], [2, 2], [3, 3]])
 
     it 'store the correct value for data', ->
       expect(@histogram.data).toEqual [[1, Math.log(2)], [2, Math.log(3)], [3, Math.log(4)]]

lms/static/coffee/src/histogram.coffee

@@ -8,6 +8,7 @@ class @Histogram
 
   calculate: ->
     for [score, count] in @rawData
+      continue if score == null
       log_count = Math.log(count + 1)
       @data.push [score, log_count]
       @xTicks.push [score, score.toString()]