fix(datepicker): update validation when switching from null to error input

[mmalerba]

fixes #12633, #11931

[crisbeto]

LGTM, can we capture this in a unit test?

[mmalerba]

Done

[AmorrowPlus]

Any idea on when this will make it in?

[sudd03]

This change introduced a new issue for date picker. The form-control updateOn blur event is not working properly.

For datepicker form control which has updateOn set to blur but after removing the date from right or left it emitting value change event.

It should emit value change event only for blur.

Here is example with @angular/material 7.3.1 version in which have issue.

https://datepicker-issue-with-angularmaterial-731.stackblitz.io

Earlier version (@angular/material 7.2.2), the blur event was properly working.

https://datepicker-issue-with-angularmaterial-722.stackblitz.io

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