Rob Kardashian resurfaces online for her sister Khloe Kardashian. The former star of “Keeping Up with the Kardashians” broke his 4-month social media silence to pen a sweet tribute to Khloe on her 39th birthday. In his rare post, Rob shared a picture of them together. “Happy Birthday @khloekardashian,” so he wrote in the caption. “Wishing you nothing but good …