修改卷库管理中，如果大题下面的小题全被删除了，再给该大题选题时，就会提示选题失败的bug

cqb-paper/src/main/java/com/qmth/cqb/paper/service/impl/PaperServiceImpl.java

@@ -639,6 +639,10 @@ public class PaperServiceImpl implements PaperService{
 				List<PaperDetailUnit> paperDetailUnits = paperDetailUnitRepo.findByPaperDetailOrderByNumber(paperDetail);
 				if(paperDetailUnits!=null && paperDetailUnits.size()>0){
 					for(PaperDetailUnit paperDetailUnit:paperDetailUnits){
+						//旧题没有难度值，需要重新赋值
+						if(paperDetailUnit.getQuestion().getDifficultyDegree()== null){
+							paperDetailUnit.getQuestion().setDifficultyDegree(0.5);
+						}
 						sum = paperDetailUnit.getScore() * paperDetailUnit.getQuestion().getDifficultyDegree() + sum;
 					}
 				}