Reproducing the problem:
1. Create a level in WED.
2. Set sunlight to {0, 0, 0}, ambient to {127, 127, 127} (or other value)
3. Build & Run - the level is entirely black, even while there is ambient lighting.
4. Add a single lightsource to level. It does not matter what parameters and position it will have.
5. Build & Run - the level works as expected now.
Conclusion:
Map compiler decides that if there are not lightsources, level does not need lightmap calculation. But that's not right, because there is ambient lighting.