As Proavia mentioned, try to blow through the tube. If you can't it is blocked. If you go ahead and drill it, likely won't be an issue and you will be happier, its your filter. If it causes an issue, open the filter and put a plug in the tube.
That filter looks very similar to a Hayward S-244T, which does not have an air-relief in it. Neither do Jandy SFTM or Pentair TA series sand filters. Sand filters with top-mounted valve are very good about getting rid of air, though some manufacturers, like Pentair do install a tube in their Triton series.
The biggest reason for an air relief is so that one is not injured on opening a tank to clean the filter. If there is enough air in the tank under pressure, the lid will blow off and seriously injure someone. Even those in the business who know better have made that mistake.
You almost never need to open a sand filter, and even then, by the time you get to the point of removing the valve any pressure would be gone as the plumbing has to be removed/opened first. Some air will always get into any filter the moment you open the pump to clean the basket. If you are having excess/continuous air you need to find and remove the source.