getBootstrapColumnPrefix(className) {
    if (className) {
      const bootstrapClass = this.getBootstrapColumnClass(className)
      if (bootstrapClass) {
        return `${bootstrapClass.split('-')[0]}-${bootstrapClass.split('-')[1]}`
      }
    }
    return ''
  }
Return the prefix (col-md)
@param {string} className
@returns {string}