Even on the equipment where you do get a graph of efficiency vs load, the line usually only starts at 10% load - that’s where your 70% efficiency figure is for.
As noted, at 0% load efficiency is by definition 0%.
It’s similar to AC-DC. From a simplistic perspective, efficiency at idle will be 0% because the converter itself still uses some power, then efficiency increases with load since the converter overhead becomes less significant as the useful work increases. Googling “dc dc converter efficiency curve” gives plenty of results.
They might be willing to spec it as "quiescent current" (current drawn at 0 load) even if they don't provide a full curve. Annoying that it's not on the datasheet.
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