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.

have you tried using (1, 1, 1) as sunlight?

Sun color vector less than (3, 3, 3) is ignored. And using (3, 3, 3) already defeats the point of not using sun.
As I've mentioned in first post, currently I'm using a 'dummy' light object.
The point is, that if there are no bugs, you do not need to use 'sideways' to overcome them.

You can read up in the manual about the special meaning of the sun color ( 0,0,0 ).