allow to set to matrix.cells a null value (clear cells): https://github.com/surveyjs/surveyjs/issues/978
allow to set to matrix.cells a null value (clear cells): https://github.com/surveyjs/surveyjs/issues/978

surveyjs/surveyjs,surveyjs/surveyjs,dmitrykurmanov/surveyjs,andrewtelnov/surveyjs,dmitrykurmanov/surveyjs,dmitrykurmanov/surveyjs,dmitrykurmanov/surveyjs,surveyjs/surveyjs,andrewtelnov/surveyjs,andrewtelnov/surveyjs,surveyjs/surveyjs