function parseLocation(city, state) {
  if (!city || !state) {
    return null;
  }

  if (city.length === 0 || state.length === 0) {
    return null;
  }

  return `${city}, ${state}`;
}
Parse separated city and state strings into a full
location if both are valid and non-null.

@param {string} city The city where the book was published.
@param {string} state The state where the book was published.
@returns The full location string.