fix(MdHighlightText): avoid printing 'null' as a value (#1352)
fix(MdHighlightText): avoid printing 'null' as a value (#1352)

fix #1351
marcosmoura/vue-material,VdustR/vue-material,vuematerial/vue-material,VdustR/vue-material,vuematerial/vue-material,marcosmoura/vue-material