feat: add optional date/datetime/interval parameter classes

[inakam]

Description

Add optional variants for all date/datetime/interval parameter classes, filling a gap in the OptionalParameterMixin coverage.

New classes (7 total):

• OptionalDateParameter
• OptionalMonthParameter
• OptionalYearParameter
• OptionalDateHourParameter
• OptionalDateMinuteParameter
• OptionalDateSecondParameter
• OptionalDateIntervalParameter

Each follows the established 3-line pattern used by existing optional parameters (e.g. OptionalIntParameter, OptionalFloatParameter):

class OptionalDateParameter(OptionalParameterMixin[datetime.date], DateParameter):
    """Class to parse optional date parameters."""
    expected_type = datetime.date

Motivation and Context

Date and datetime parameters are the only built-in parameter family without optional variants. Users who need optional date parameters must define their own subclass, despite OptionalParameterMixin being designed for exactly this purpose. See existing optional parameter additions in #3406.

Have you tested this?

I have included unit tests. All 7 new parameter classes are tested for:

• Parsing empty strings to None
• Parsing valid date/datetime strings to correct types
• Serializing None to empty string
• next_in_enumeration returning None
• Building tasks with config values (both default=None and explicit defaults)

[dlstadther]

@inakam , does this introduce parameter types you require? Or is it just to contribute towards "completeness"? I'm not inclined to add parameter types if there aren't explicit requested uses of them.