Fix selection when value is empty string (#2075)

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 || this.value === false)
+ let isSet = (this.value || this.value === 0 || this.value === false || this.value === '')
  return isSet ? this.value : this.uniqueId
  },
  inputLabel () {