How to reproduce:
1. Open the attached project ("case_1157273-IsEmptyTrackNotWorking.zip")
2. Open the repro scene ("SampleScene")
3. Open the Timeline window and inspect the "Director" object in the Hierarchy
4. Try scrubbing the timeline and see the console logs

Expected results: Empty track is included in the graph as it has "isEmpty" property overridden to "false"
Actual results: Empty track is not included in the graph

Reproducible with: 2019.1.5f1, 2019.2.0b5, 2019.3.0a5
Couldn't test with 2017.4 and 2018.4 as it does not support Timeline package
Reproducible with package versions: 1.0.0, 1.1.0