code: function strArray(array) {
    if (Array.isArray(array)) {
      array = array.filter(isString);
      return array.length ? array : null;
    }
    return isString(array) ? [array] : null;
  }
docs: Coerce the input to an array of strings. If the input is a string, wrap it in
  an array. If the input is a string, filter out all the non-string elements.
  If we end up with an empty array, return null.
  
  @param {*} array
  @return {?Array<String>} The output array.