Reproduction steps:
1. Open the attached project "onCurveWasModified 2018.2.zip";
2. Open the Animation Window;
3. Select an empty Animation Clip that does not hold curves in Project;
4. Change 'Samples' of Animation Window;

Actual results: You will see that Log of typeof (Object) 'CurveModified' has been called.
Expected results: only 'ClipModified' should be called.

Reproduced on: 2018.1.0b1, 2018.1.4f1, 2018.2.0b8, 2018.3.0a1.
Not reproduced on: 2018.1.0a7, 2017.4.5f1 or lower.