fix(MdField): Consider number 0 as hasValue (#1407)

fix #1406

Author: VdustR

src/components/MdField/MdField.vue

@@ -70,7 +70,7 @@
     },
     computed: {
       stringValue () {
-        return this.MdField.value && this.MdField.value.toString()
+        return (this.MdField.value || this.MdField.value === 0) && this.MdField.value.toString()
       },
       hasCounter () {
         return this.mdCounter && (this.MdField.maxlength || this.MdField.counter)