Setting Scale in lightmap to 0 on an object removes it from the lighting calculation in 2017, and does not produce a realtime lightmap.
In 5.6 the object is still affecting the scene and still produces a realtime lightmap when scale in lightmap is set to 0

Expected: Emissive object still affects the scene and realtime lightmap is produced for the object.

2. How we can reproduce it using the example you attached

1. Open the project 'Scale in lightmap' in 2017.x
2. Open the scene 'Scale in lightmap'
3. Let the autobake complete
4. Observe the scene is unlit and not affected by the emissive object
5. Open the same scene in 5.6
6. Observe the emissive object is lighting up the scene

Occurs in : 2017.x

Does not occur in 5.6.2f1