assert proper array access for _dateTimeSelected()

I extend the FormHelper for some additional markup.
While using the inputs date/datetime I get "Warning (2): strlen() expects parameter 1 to be string, array given" when the array is not complete (contains only year, not month/day etc).
This happens since the isset() is on the same level as the is_array() and it can run into the scalar checks afterwards in the else block.
This fixes it.