Fix selection when value is empty string

Use single quote

Author: ittus

src/components/MdField/MdSelect/MdOption.vue

@@ -39,7 +39,7 @@
         return this.MdOptgroup.disabled || this.disabled
       },
       key () {
-        let isSet = (this.value || this.value === 0)
+        let isSet = (this.value || this.value === 0 || this.value === '')
         return isSet ? this.value : this.uniqueId
       },
       inputLabel () {