LukasBoll changed the title WIP: Fix/datepicker validation Fix/datepicker validation

LukasBoll changed the title Fix/datepicker validation WIP: Fix/datepicker validation

LukasBoll changed the title WIP: Fix/datepicker validation Fix/datepicker validation

Previously, data wasn't cleared when the input field was emptied.
This commit will set the value to 'undefined' when the input is empty or
invalid upon blurring. During editing, the data is only updated when the
current input is valid.

Closes eclipsesource#2183