JsonAnnouncement's RecordDate should be able to parse to null.

[KevinDeBeer]

Description

Currently GetAnnouncementAsync and ListAnnouncementsAsync might fail when record_date is null.
In that case, an unhandled exception occurs.
Checking the API reference on https://paper-api.alpaca.markets/v2/corporate_actions/announcements shows that record_date is returned as null quite often.

Fixes # (657)

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Reproduction:
Execute ListAnnouncementsAsync with a StartDate of 2023-08-01 and EndDate of 2023-08-20.

Code sample:

var actionTypes = new CorporateActionType[] { CorporateActionType.Dividend, CorporateActionType.Merger, CorporateActionType.SpinOff, CorporateActionType.Split };
var interval = new Interval<DateOnly>(DateOnly.FromDateTime(startDate), DateOnly.FromDateTime(endDate));
var announcementsRequest = new AnnouncementsRequest(actionTypes.ToList(), interval);
var announcements = await _tradingClient.ListAnnouncementsAsync(announcementsRequest, cancellationToken);

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings

[OlegRa]

LGTM. Thanks a lot for this fix!