1. First, give yourself a valid spawn egg for summoning a sand entity. I used this one so it looked nice and also functioned as a sand entity spawn egg:

   /give @p minecraft:cod_spawn_egg{EntityTag:{id:"minecraft:falling_block",BlockState:{Name:"minecraft:sand"},Time:1b,CustomName:""},display:{Name:"{\"text\":\"Falling Sand Spawn Egg\",\"italic\":false}"}}
   

2. Use it in creative mode to verify it works. you should spawn a block of sand.
3. Put the same egg into a dispenser and power it with redstone. 

The dispenser strips the entity of all the rest of the data other than the fact its a falling_block entity. This can be further tested by changing the block id to spawn to a block other than sand; and seeing that the dispenser changes it to a falling sand instead of what you defined.

Note: this has nothing to do with spawners or the spawn eggs themselves, as the eggs work perfectly fine, and all entities excpet falling_block seem to work just fine when being dispensed by a dispenser.