we should keep the input value for cf date/datetime

without that, we may lose its value if first "submit" fails.
see #19660
allow to delete a cf date value

as now empty cf date input means delete only
keep values of ticket core date fields too
Remove unused Value argument to SelectDate

8fea92b added Value to the arguments.  However, the issues ticket it
references claims that it's so Ticket/Create.html is sticky -- but as of
8fea92b, Ticket/Create.html was passing SelectDate Default =>, not Value
=>.  Ticket/Create.html, the one call site which passed Value =>, was
fixed to use the standard Default in a00a733, prior to 8fea92b.
check if it's necessary to add custom field value before actually adding it.

if a user inputs 0 as a Date cf value, we should not add it as the new value,
since it means "not set", as it does in ticket's core date fields.
Merge branch '4.4/date-input-keep-value'