Steps to reproduce:
1. Open attached project "Case_1089973-HandlesScaleValue.zip"
2. Press on "SomeValueNotZero" GameObject
3. Try to move the white dot which changes "Some value" float and Observe that value changes in the Inspector window
4. Now press on "SomeValueIsZero" GameObject (which has its start value set to 0)
5. Try to move the white dot again

Expected result: White dot moves and "Some Value" float changes.
Actual result: White dot doesn't move and value does not change.

Reproduced in 2017.4.16f1, 2018.1.9f2, 2018.2.18f1, 2018.3.0b12, 2019.1.0a10

Note: It looks like it's scaling the handle speed by its current value. It will make the handle go faster the higher the value is and slower the lower the value is. At minus described bug occurs and on the minus value, handle starts moving in the opposite direction than your mouse.