fix Error tooltip not displayed in a datepicker

It happens when the datepicker is not initialized yet and an invalid value is entered : null is compared to undefined, resetting the DP state
I wouldn't use a specific name such as `startDate`, since for someone not knowing what the test should do, one might expect something like `endDate` somewhere. Generally speaking, don't use specific naming with particular semantics if it's not used as is afterwards, it can be confusing, especially to understand the purpose of the code.
I would rename it simply to `date`.
This means renaming other references to it especially in associated module controller.