Stack trace when exporting assessment results which have unanswered calculated questions

Steps to reproduce the issue:

1. The instructor publishes an assessment which have any calculated question.
2. The student takes the assessment and submits it without answers the calculated question.
3. The instructor try to export the assessment results by clicking the "Export" button. Samigo throws the next stack trace in the application logs and the export doesn't work.

SEVERE: java.lang.NullPointerException
at org.sakaiproject.tool.assessment.ui.listener.evaluation.HistogramListener.getCalculatedQuestionScores(HistogramListener.java:1830)
at org.sakaiproject.tool.assessment.ui.listener.evaluation.HistogramListener.doAnswerStatistics(HistogramListener.java:906)
at org.sakaiproject.tool.assessment.ui.listener.evaluation.HistogramListener.determineResults(HistogramListener.java:812)
at org.sakaiproject.tool.assessment.ui.listener.evaluation.HistogramListener.histogramScores(HistogramListener.java:457)
at org.sakaiproject.tool.assessment.ui.listener.evaluation.HistogramListener.getDetailedStatisticsSpreadsheetData(HistogramListener.java:2879)
at org.sakaiproject.tool.assessment.ui.bean.evaluation.ExportResponsesBean.getSpreadsheetData(ExportResponsesBean.java:181)
at org.sakaiproject.tool.assessment.ui.bean.evaluation.ExportResponsesBean.exportExcel(ExportResponsesBean.java:172)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ...