getOptionLabel (option) {
      if (isEmpty(option)) return ''
      /* istanbul ignore else */
      if (option.isTag) return option.label
      /* istanbul ignore else */
      if (option.$isLabel) return option.$groupLabel

      const label = this.customLabel(option, this.label);
      /* istanbul ignore else */
      if (isEmpty(label)) return ''
      return label
    }
Returns empty string when options is null/undefined
Returns tag query if option is tag.
Returns the customLabel() results and casts it to string.

@param  {Object||String||Integer} Passed option
@returns {Object||String}