fix: extendend content handling srcdat field for unsupported srcdat field values

[crisrivlop]

Changes

Add support for Null dates when extendedContent.ts/uploadMemberContentWithDates function is called.

How to test this PR

Examples:

1. Open a remote file with SRCDAT field in null if extendedMemberSupport property is true. It should open with this new change.

Checklist

• have tested my change
• have created one or more test cases
• updated relevant documentation
• Remove any/all console.logs I added
• have added myself to the contributors' list in CONTRIBUTING.md

[sebjulliand]

I like the idea, but try_cast has been introduced in 7.5 and 7.4 TR6 - and for now, Code for IBM i still supports 7.3 TR 5 at a minimum.
This won't make it's way into the extension until the minimum OS version we supports becomes 7.4 TR6 (which would prevent quite a lot of people from using the extension, sadly).

[worksofliam]

@sebjulliand you are fully right. I also worry about the performance implications of this additional function call. This process is already a little bit slow.