The late Betty White was a tireless advocate for animals for her entire life, from caring for homeless animals as a child to launching her own weekly TV show “The Pet Set” dedicated to her celebrity friends and their pets. Her biggest contribution, though, may be yet to come, as fans get set to donate to animal welfare charities and local shelters on Jan. 17 as part of the #BettyWhiteChallenge. And animal welfare nonprofits are already gearing up to make the most of the viral tribute to the star of “The Golden Girls” and “Hot in Cleveland” on what would have been her 100th birthday.