async function saveOrDestroyVariable(key, value) {
  if (value === undefined || value === null) {
    await this.gladys.variable.destroy(key, this.serviceId);
  } else {
    await this.gladys.variable.setValue(key, value, this.serviceId);
  }
}
@description Save or destroy Gladys variable.
@param {string} key - The unique key of the variable.
@param {string} value - The value of the variable.
@example
await this.saveOrDestroyVariable('key', 'value');