While it's actually not too hard to get empty photos of Grand Central Terminal (since the terminal closes from 2am-5am), it's definitely unusual to find it so empty during rush hour.
Below are some side by side comparisons of Grand Central Terminal. Left photos are from December 29, 2019 at around 6:30PM and the right photos are from March 19, 2020 at around 6PM.
Some additional photos from March 19, 2020
Of course, this is a good sign since it means most people are staying home (unlike me, who still has to go in to work at least twice a week... )