What's a zero-pointer?
Basically a player is normally allowed to count 4 ATP500 results (or Monte-Carlo) : if he gets a zero-pointer, one of these results is a "zero" and then he can actually count only 3 of them.
A player who withdraws from an ATP500 tournament (or Monte-Carlo) who is not out for 30 days because of a long-term injury, and who doesn't come on-site to show his injury or make some promotion (and who didn't get injured on friday to sunday in another tournament previous week like Gasquet in Zagreb),
normally gets a zero-pointer.
Djokovic didn't fly to Rotterdam to make some promotion or have his injury checked from what we heard and Murray probably doesn't intend to go to Dubai either.
In an ATP500 tournament (contrary to a MS1000 tournament), it's different from losing in first round of the tournament, because in that latter case you can replace this result with a result in another ATP500 tournament.